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CLASSiC DAC Pt.4 Final article gives the test procedure and describes how it’s used By NICHOLAS N ICHOLAS V VINEN INEN Having described how the new high-performance CLASSiC DAC works and shown how to build it, it’s now time to set it up & put it through its paces. We’ll also go into some details about how to use it, especially the more advanced features. W e left off last month having assembled the CLASSiC DAC PCBs and connected them together in the case. Before firing it up, you will need to program the microcontroller, if it wasn’t supplied programmed. Before doing that, it’s a good idea to do some quick checks to ensure that the power supply is working and delivering the correct voltages. This will avoid damaging the ICs when power is applied to the rest of the circuit. Test procedure Before applying power, check that: IC4 and IC6 are out of their sockets; JP1 is not fitted; JP2 and JP3 are fitted in the 0dB positions; • VR1, VR2 and VR3 are rotated fully anti-clockwise; • LK1 and LK2 (closely-spaced pairs of pads labelled in red on the overlay diagram) are clear of solder; • All the DIP switches are off; • • • siliconchip.com.au • Your DMM is set to measure DC volts; and • You have the PCB orientated as shown in the overlay diagram last month, with the connectors at left, so that you can easily follow the instructions below With a DMM at the ready, connect a 9VAC plugpack and switch it on, then measure the output of REG3 (bottommost pin) using its tab mounting screw as the ground reference point. You should get a reading of around 5V (4.8-5.2V). If not, switch off and check for faults. Also measure the output of REG5, the 4-pin SMD to the left of bobbin inductor L6; use the same ground reference point as before and carefully probe its top-most pin. You should get a reading very close to 3.3V. If this is not between 3.2V and 3.4V, switch off and check the board carefully, especially in the power supply (lower left) area. Assuming it’s OK so far, check the ±15V outputs from REG1 and REG2. Like REG3, their outputs are the bottom-most pin and for REG1, this should read around 15V (14.5-15.5V say). When checking REG2, be aware that you can’t use its mounting screw as a ground reference since that is actually connected to the regulator input. You should get a negative voltage in a similar range as that indicated for REG1. Finally, check the input to REG5, ie, its bottom-most pin (we measured its 3.3V output earlier). You should get a reading just below 4V. If it’s much higher than that, it could be that REG4 is not operating correctly and this would lead to REG5 overheating during operation so switch off and check for faults around REG4. If you get the correct reading, switch off anyway as the power supply tests are finished. At this point we should say that when we ran through these tests for May 2013  57 Setting Up The Remote Control A universal remote control can be used to control the DAC and is virtually mandatory if you will be using the SD card playback capability. Before the remote can be used, it must be set to send out the right codes. For the Altronics/Dynalink A1012 remote, put it in TV mode and use code 156. To do this, press “TV” while holding down the “Set” button, then enter 156 on the keypad. If this conflicts with other equipment you own, slide DIP switch #3 on the PCB to its on position, then set the remote to TV code 170 instead. For the Jaycar/Digitech AR-1726 re- our prototypes, in two cases the 7915’s (REG2) output was much higher than expected at around -20V. This was fixed by replacing the regulator with an On Semiconductor branded unit which we got at our local parts shop. We think that the batch of 7915s we had in stock were dodgy but it’s possible that this is a widespread problem with certain brands of regulator under these conditions (input voltage close to maximum, output current draw low). While the -20V would probably have dropped quickly once a load was applied, rather than risk damage to the op amps, we elected to replace the regulator and suggest you do the same if this happens to you. Making repairs If you do have to de-solder a component to replace it, be careful as the plated-through holes really hold onto the pins well and you don’t want to damage the PCB. For TO-220 package parts like REG2, the best method is to remove the screw, bend the tab up to vertical, hold the tab with pliers and heat all three pads simultaneously (add solder if necessary) while gently pulling the part away from the board. If it doesn’t come out a few seconds after all the solder has melted, wiggle the tab from side-to-side. If it still won’t come out, let it cool down and try again later, to avoid lifting any pads from excessive heat application. Smaller components can be removed using a similar technique although it’s usually easier to cut one or more of the leads off first, remove 58  Silicon Chip mote, use code 252. To enter this code, hold down the mode button you want to assign (TV, VCR, AUX, etc – it doesn’t matter which) for a few seconds. Then enter the code, 252, press OK and after a couple of seconds, press the mode button again. If for some reason this doesn’t work for you and you want an alternative, change the position of DIP switch #3 to on and use code 281. Verify that the remote is working by changing inputs with the numeric buttons 1-8. You should also be able to turn the DAC on and off (into/out of standby the body of the component and then de-solder the leads individually. A solder sucker can then be used to clear the holes so that a new component can be fitted. Next steps Now short both LK1 and LK2 with solder. Be careful not to touch any adjacent components with the soldering iron. You may have to try a few times before you manage to successfully bridge the pads since they are quite small. Use a DMM to check that there is continuity between the output pin of REG3 (bottom) and the lower pin of JP1. Check also for continuity between the top-most pin of REG5 and the upper pin of JP1. Now plug IC4 and IC6 into their sockets, checking the required orientation carefully (they’re different). If you haven’t already connected the main board to the front panel with the two cables made earlier, do so now. Slide the top-most DIP switch (Power On) to the On position and fit a shunt on JP1 in the appropriate position. Most TOSLINK receivers will operate from 3.3V these days but it’s best to check with the supplier if unsure. Programming the micro If you built the DAC from a kit or are using one of our pre-programmed microcontrollers, it should be ready to go. If not, you will need to plug a pin header into CON11’s pads on the PCB; it should be a tight fit and will be held in by friction. Connect a PICkit3 or similar in-circuit programmer and The unit will work with most universal remotes including the Altronics A1012. mode) using the power button and change the volume using the volume up/down buttons. The power switch LED will flash to acknowledge the reception of infrared commands. flash IC5. The HEX file is available from the SILICON CHIP website. You can use the PICkit3 to supply 3.3V for the microcontroller while flashing. We have confirmed that this works OK. But if that isn’t possible for some reason, you can connect the plugpack instead. Once MPLAB is configured for the correct device (dsPIC33FJ128GP306), the ICSP is connected and power is applied. The chip should then be recognised and its revision number shown. If not, switch off and check for faults, especially in IC5’s solder joints. You may also find, as we did on one of our prototypes, that the friction-fit header makes marginal contact and you have to put pressure on it to get a reliable connection. In this case you can simply solder the ICSP header (CON11) to the PCB. Once the chip has been programm­ed, you should get a message that it was successfully verified. You can then switch off, disconnect the programmer and unplug CON11, assuming you haven’t soldered the latter in place. Checking the LEDs With power applied, confirm that the power LED and the Input 1 LED light up. A brief press of the power button should cycle through the available inputs and let you check that all eight input LEDs are working OK and have a reasonably similar brightness level. Because we’re running the blue LEDs at a low current (to avoid frying your eyeballs in a dimly lit room), you may find that there are some “duds”. We had a few of these in the batch of siliconchip.com.au LEDs we bought and ended up replacing several on the PCB. If you measure the resistance between anode and cathode, you will probably find any duds will measure below 1kΩ in one or both directions. This shunt resistance is a manufacturing fault; the LEDs probably work OK at higher currents so you can use them for something else. Even if you don’t have any duds, the LED brightness might be uneven. We have tried to compensate for the different efficiency of green, blue, yellow and red LEDs by varying the current limiting resistors but your LEDs may have a different brightness/efficiency than ours. This won’t usually matter but if they vary wildly, you may want to shunt some of the limiting resistors or possibly even replace them to get a more even display; not that it’s all that critical as they are normally only lit up one at a time. Setting quiescent current & gain Now set up the headphone amplifier. Measure the voltage across the top-most 22Ω emitter resistor in the upper-right corner of the board, next to ZD7. There are a pair of adjacent pads connected in parallel with this resistor that you can use to connect the probes. You should get a very low reading, ie, just a few millivolts. Now slowly turn VR2 clockwise until you get a reading of 0.1V. This should be with VR2 rotated roughly half-way through its full travel. Then measure the voltage across one of the other 22Ω resistors immediately behind the headphone jack socket and adjust VR3 to get the same reading. If you are planning to use high impedance headphones (>60Ω) then you will probably want to set the headphone output gain to +12dB. To do this, move JP2 and JP3 to their alternative positions. Final testing & set-up The final test is to check that the sound output is working correctly. This is done most easily with a WAV file loaded on an SD card although there are other methods (see below). Load a WAV file copied from a CD (ie, 44.1kHz 16-bit stereo) onto a blank SD card and plug it in to the DAC. Connect headphones (using a 3.5mm to 6.5mm adaptor if necessary) and turn the volume down close to minimum. If you don’t have headphones, you siliconchip.com.au Fig.14: when the DAC is connected to a computer’s USB port, it will be detected regardless of whether the DAC itself has power. This shows how it appears in Device Manager under Windows 7 – as “USB Audio CODEC” under “Sound, video and game controllers”. You can check that it is the default output device in the Sound control panel and if not, make it the default. can hook up an amplifier to the line outputs instead. Power up and check that LED8 (red) turns on, dimly at first and then brighter once the SD card has been initialised. If you now slowly turn up the volume, you should hear the file being played back. Check that it is in stereo and distortion-free. If LED8 doesn’t light or stays dim, check the soldering on the SD card socket and the microcontroller. It could also be a problem with the components behind the SD card socket. There’s also a possibility that the WAV file format is incorrect; while the DAC will play most WAV files with a supported sampling rate, bit depth and number of channels, the WAV format allows for quite a bit of variation and some files may not be properly recognised. So it might help to reformat the SD card and load a different WAV file on. If that went well, it shows that a large portion of the circuit is working correctly. If you can’t get the file to play from the SD card though, or you don’t have a suitable card, try one of the following methods to help narrow down the problem. into a computer using an appropriate cable (Type A to Type B). It should be recognised and a new audio interface should appear (the DAC’s USB interface runs off the computer’s power supply). Fig.14 shows how the device shows up in Windows 7 Device Manager (as “USB Audio CODEC”) and in the Sound settings in the Control Panel. In most cases, your computer will automatically select the USB audio device as the currently active sound output. If it doesn’t, you will then have to make it the current output device. Once you’ve verified that it is active, switch the DAC power on and play some audio from the computer. It should sound clear and undistorted. If the SD card output works but USB audio doesn’t, check the circuitry around IC2. The most likely symptom if there is a fault in this circuitry is that the USB Audio CODEC will not be detected by your computer. Alternatively, if USB audio works but playback from the SD card doesn’t, this suggests either the DAC doesn’t recognise the file format or there is a problem in the circuitry associated with the SD card. Testing the USB interface Finally, if you have a TOSLINK or S/PDIF digital audio source, check Power down the DAC and plug it Testing the other inputs May 2013  59 Equal Loudness Curves ISO 226:2003 120 100 10dB 80 phon 80 60 phon 18dB 60 20dB 20 phon 40 40 20 phon 20 0 phon (threshold) 0 20 50 that the other inputs are working correctly. The associated input LED should light up dimly when a signal source is plugged into each input and you can then use brief presses of the power button to cycle the current input until the one being tested is selected. You can then check that the audio output is clean. Having tested that all the inputs are working (or at least, those which you can) and that the headphone output is OK, that verifies that virtually all the circuit is operating correctly. Setting the DIP switches The “Setting Up The Remote Control” panel (page 58) discusses the use of DIP switch #3 to select the alternative set of remote control commands. DIP switch #1 determines whether the unit switches on initially when power is applied; if set to off, the DAC will start up in standby mode and must be switched on before use by pressing the power button on the unit itself or on the remote control. If the second DIP switch is left off, the unit will only change inputs automatically when an SD card is inserted or when the unit is plugged into a computer via USB. In both these cases, it automatically switches to the relevant input. With DIP switch #2 on, however, it will also switch to another input if a valid signal appears on that input and either there is no signal on the current input or there has been no audio on the current input for at 60  Silicon Chip (estimated) Fletcher-Munson 100 phon Sound Pressure Level (dB SPL) Fig.15: each red line plots the sound pressure level required to give sounds that appear to have equal loudness over the range of audible frequencies, from 20Hz up to nearly 20kHz. The lines are plotted for 0, 20, 40, 60, 80 and 100 phon where 1 phon = 1 dB SPL at 1kHz. The auto-loudness feature compensates for the ear’s loss in sensitivity at low frequencies by progressively boosting the bass as the volume level is reduced (see text). 100 200 500 1k 2k Frequency (Hz) 5k 10k 20k least 10 seconds (this time period can be changed via the configuration file; see below). Finally, the fourth DIP switch controls whether the power LED is lit dimly when the unit is in standby (on) or not lit at all (off). Using the DAC At this stage, if you just want to use the unit as a plain DAC, you should be ready to go. When on, it can be switched into standby by holding the power button down for a second or so, or by pressing the on/off button on the remote control. A brief press of the power button switches it back on. The current input can be changed by further presses on the power button or via the numeric buttons 1-8 on the remote control. The mute button on the remote control can be used to temporarily mute the output while the volume buttons will change the volume digitally. However for best quality, leave the digital volume at or near maximum and adjust the volume on the headphone volume pot or, when using an external amplifier, with that amplifier’s volume control. Most of the remaining controls are useful mainly when playing back WAV files from an SD card, ie, when using the CLASSiC DAC as a music or audio player. SD card playback If you have more than a few audio files on an SD card, then typically these should be arranged in folders (aka directories). For example, if playing music, you could have one folder for each musician, with another set of folders within these for each album or disc. The WAV files would then be within these folders. Alternatively, you could simply have one folder in the root directory for each disc, with its name prefixed by the artist/performer. Both schemes will give a similar result. If you want the tracks to play back in the same order as they are on the CD then the usual practice is to prefix each track with its number, padded out to two digits. For example, the first could be named “01 – Track One.wav” or “01 Track One.wav”. That way, when sorting the file names alphabetically (which is the default), they will be played in the correct order. With a card inserted, use the left and right arrows on the remote control to skip between tracks within a folder. Skipping past the beginning or end will initiate playback from the previous/next folder. Similarly, if you press the left arrow when playing the first track of the first folder it will “wrap around” to play the last track of the last folder. Conversely, if you press the right arrow on this last track, it will wrap in the other direction. The down arrow will skip to the first file in the next folder regardless of which track is being played from the current folder and similarly, the up arrow will go to the first file in the previous folder. This is roughly equivalent to “next/previous disc” with a CD changer. So with either of the folder schemes mentioned above, the up and down arrows will move between the discs from a given artist or performer and if you continue pressing these keys, you will switch to the (alphabetically) next/previous artist. Navigating the tracks While there’s no display to show you which track is currently being played (ie, you’re navigating “blind”), if you have a list of the folders on the card then it is usually a fairly simple matter to navigate to the one that you wish to use. For reference, a 16GB SD card will fit around 23 full CDs worth of WAV files, a 32GB card 45 CDs and a 64GB card at least 91 CDs worth. In practice siliconchip.com.au you will fit more since CDs are rarely full (ie, the track total is usually less than 80 minutes). Button Function Changing file order Power Enter/leave standby It’s also possible to play tracks from the card in a different order. If you press the “OK” button on the remote then the playback order changes from alphabetically sorted to shuffled. Then when you change to the next/previous track, it will jump to a random track in the current folder. The order of folders is also shuffled, so pressing the up/down arrows will go to a random folder within the current nesting level. Pressing the OK button again then changes the playback order so that the tracks play in the order that they are stored on the SD card. This will tend to be the order in which you loaded the folders and files onto it, although that isn’t guaranteed. A third press of the OK button returns to the original alphabetical sort. Mute Turn mute on or off 1-8 Switch to input 1-8 Pause (WAV) Pause/resume Rewind/Fast forward (WAV) Skip back/forward 10 seconds Play/Stop (WAV) Start/stop playback TV/Video (A1012) / Menu (AR1726) Go to control mode 1 (normal) Teletext (A1012) / TV/VCR (AR1726) Go to control mode 2 (tone controls) Page hold (A1012) / TV/AV (AR1726) Go to control mode 3 (crossfeed control) Tone controls The digital tone controls (bass/treble boost/cut) operate only when playing back WAV files from an SD card. This is because we wanted to avoid having audio pass through the microcontroller if it wasn’t necessary, as the sound quality could potentially be affected even though it is digitally transmitted (eg, due to added jitter). However, since the microcontroller is in charge of WAV playback anyway, we have taken advantage of the ability to do some extra processing. Table 1: Remote Control Buttons With Same Function In All Modes This feature is limited to sampling rates of 48kHz and below because the microcontroller simply isn’t fast enough to play back higher sampling rate WAV files and process them too. It’s automatically disabled at higher sampling rates. You can boost or cut the treble or bass by a few decibels. Since this is done digitally, it has relatively little impact on sound quality although there is inevitably some loss in dynamic range. By default, the bass boost/cut is affected with a first-order low-pass filter that has a corner frequency of 500Hz. Similarly, treble boost/cut uses a firstorder high-pass filter with a -3dB point of 2kHz. You can adjust these frequencies within a limited range although there’s little benefit in doing so. Briefly, to use the tone controls: • Press the “teletext”* (A1012) or TV/ VCR (AR1726) button to enter control mode 2 (* = see Table 1). • Press the record button to enable the tone controls. • Use the up/down/left/right buttons to adjust treble and bass levels. Note that with the AR1726 remote, these may not automatically repeat when held down. • If necessary, use the volume up/ down and channel up/down buttons to adjust the corner frequencies. • Press the TV/Video (A1012) or Menu (AR1726) button to switch the remote functions back to default. Note that as you adjust the tone controls, the volume automatically changes to avoid clipping the audio signal. This interferes with the ability to compare the sound with and without tone controls so you can press the 0 (zero) button while in control mode 2 to reduce the audio volume Digital Tone Control Implementation As described in the main text, the CLASSiC DAC can provide digital tone controls when playing back WAV files. This involves manipulating the digital audio data after it has been read off the SD card and before it is send out as an S/PDIF stream to eventually reach the DAC IC. To do this, we are using a digital signal processing (DSP) technique known as an Infinite Impulse Response (IIR) filter. This is effectively a digital version of the analog RC low-pass/high-pass filter although IIR can also be used to build the digital equivalent of more complex filters too. The name “infinite impulse response” siliconchip.com.au comes from the fact that with an IIR filter of sufficient precision, a transient (impulse) will have an exponentially decaying effect on the output of the filter that never quite ends although practical limits of numeric precision mean that its influence will eventually reach zero. This is as opposed to Finite Impulse Response (FIR) filters which have a defined “window” so each impulse only affects a limited number of audio samples before/ after that impulse. The only numerically intensive part of implementing an IIR filter is calculating the filter coefficients, which can be done once for a given combination of corner frequency, sampling rate and gain. The CLASSiC DAC firmware does these calculations using floating point arithmetic. This is slow but because it’s only done when the filter or sampling rate changes, that doesn’t matter. During playback, each IIR filter only requires a few integer multiplication operations (2-3) and a summation per audio sample. For more information on how to build a simple digital IIR filter, see the following page: http://freeverb3.sourceforge. net/iir_filter.shtml The low-pass filter in the crossfeed routine also uses an IIR algorithm but this is combined with a small delay buffer in order to create the desired effect. May 2013  61 Table 2: Mode-Specific Remote Control Buttons Button Function In Control Mode 1 Function In Control Mode 2 Function In Control Mode 3 ChUp/ChDn Pan left/right Change bass -3dB frequency Change crossfeed attenuation VolUp/VolDn Change volume Change treble -3dB frequency Change volume Left/right arrow Previous/next WAV file Change treble gain Change crossfeed LPF frequency Up/down arrow Previous/next WAV folder Change bass gain Change crossfeed delay OK Change playback order Reset tone controls Reset crossfeed settings Record Pan to centre Toggle enable tone controls Toggle enable crossfeed 0 - Tone control equal volume mode - 9 - Enable auto-loudness - regardless of the tone control settings. You can then use the record button to toggle the tone controls on and off for an A/B comparison. Note that you will probably need to increase the headphone/amplifier volume in this mode. Pressing zero again toggles it off (and increases the volume again), which you should do when you’re finished as this gives better audio quality. Auto-loudness You may have noticed that music normally sounds better when it’s loud. It’s because the human ear’s frequency response is different at different sound levels. As you lower the volume of the music, keeping its frequency distribution even, the bass (and to a lesser extent, the treble) seem to “drop out”. The result is that the music sounds “thin” at lower volume levels. To see why this is, have a look at the accompanying graph, Fig.15. This shows both the original equal-loudness curves (“Fletcher-Munson”) and the more up-to-date and accurate ISO 226-2003 curves (reproduced from Wikipedia). Consider the red line showing the 60 phon level (a measure of apparent loudness). At 1kHz, the sound level must drop 20dB for an apparent reduction in loudness of 20 phon. But if you follow the red 60 phon line down to 30Hz, the drop to the 40 phon level is just 10dB (from 100dB SPL to 90dB SPL). Hence a drop of 20dB at 30Hz reduces the apparent sound level by about 40dB. By boosting bass at lower volumes, we can compensate for this. The CLASSiC DAC’s auto-loudness feature does just this; it also boosts treble at low volumes but less so than bass. Some old amplifiers had this 62  Silicon Chip type of feature (in analog, naturally) but it is now rare. We found that the DAC implementation of loudness compensation works quite well and greatly increases listening enjoyment at low volume levels. This feature works by taking over the tone controls and adjusting them automatically as the digital volume level is adjusted. As such, it has the same restrictions outlined above. In control mode 2, enable the tone controls and then press the 9 button on the remote to enable this mode. Making any tone adjustment immediately disables it. With auto-loudness enabled, switch back to control mode 1 as described above and turn the volume down to hear the effect. Crossfeed This is a feature for headphone users. You may have noticed that headphones can sound unnatural, especially in recordings with wide stereo separation – the sound you are listening to can appear to come from inside your head and move around inside it! This is because the headphones effectively provide totally separate sounds to each ear but normally, you can hear a sound in both ears even if it’s coming from one side of your head. The sound waves still make it to the other ear, although attenuated. Crossfeed simulates this by delaying the sound for each channel, attenuating it, applying a low-pass filter and then feeding it across to the other ear (ie, “crossfeed”). The effect of the crossfeed feature is quite subtle but can be enough to remove the unnerving aspects of the stereo signal. As with the tone controls and auto-loudness, this only applies to WAV file playback at sampling rates up to 48kHz. To turn it on, press the “page hold”* (A1012) or TV/AV (AR1726) button and then press the OSD button. Another press of this button turns it back off so you can compare the sound with and without crossfeed (* = see Table 1). Note though that the sound volume will drop slightly with it on, for the same reason as this happens with tone controls – ie, to avoid the signal clipping. You can combine crossfeed and auto-loudness or tone controls, if you wish. The various parameters for this mode can be adjusted but the defaults chosen should work well for most people (see the list of remote control commands for the list of parameters that can be changed). Pressing the OK button while in control mode 3 will reset these changes while the TV/ Video or Menu button can be used to get back to normal remote control mode as explained earlier (crossfeed will stay on if enabled). Since crossfeed is only really useful when using headphones, it is automatically disabled if there is nothing plugged into the headphone socket. You can override this in the configuration file though (eg, if using an external headphone amplifier) – see below. More remote control commands Tables 1 and 2 show all the remote control commands. Many of these have already been explained. Note that some buttons change function depending on the current control mode; the DAC starts in control mode 1 but you can then switch it to 2 and 3 using the buttons indicated. Some of the buttons on the A1012 remote are labelled with pictograms which many people won’t recognise so we have put small images of these siliconchip.com.au Table 3: Standard Settings Setting Name Possible Values Input 1, 2, 3, 4, 5, 6, 7, 8 Mute true, false Volume 0 to 255 (1 = -0.5dB, 2 = -1dB, 3 = -1.5dB etc; default = 0) Balance -20 to 20 (-20 = fully left, default = 0) Init_File <name of file to play first when card is inserted> PlaybackOrder sorted, directory, shuffle Auto_Play true or false (default = true) Auto_Switch_Enabled false, true (overrides DIP switch 2) Auto_Switch_Delay 1-999 (seconds; default = 10) AlternativeIRCodes false, true (overrides DIP switch 3) Tone_Enabled false, true Tone_EqualVolume false, true Tone_AutoLoudness false, true Tone_BassBoostCut -16 to 16 (-16 = full cut, 16 = full boost) Tone_BassCrossoverFreq 50 to 950 (in Hz; default = 500) Tone_TrebleBoostCut -16 to 16 (-16 = full cut, 16 = full boost) Tone_TrebleCrossoverFreq 1000 to 5000 (in Hz; default = 2000) Crossfeed_Enabled false, true Crossfeed_LPFFreq 50 to 5000 (in Hz; default = 1500) Crossfeed_Delay 1 to 32 (in samples; default = 14 for about 0.5ms) Crossfeed_Atten 1 to 5 (effect is halved as number increases; default = 3) Table 4: Variable Extra Settings (All Are True/False; Default = False) Setting Function StereoSwap Swaps left & right audio channels DownmixToMono If true, output is a mono mix of left & right channels Filter_Slowrolloff Selects alternative DAC output filter (not recommended) InvertPolarity If true, output phase is inverted NoFreeRunningPLL If true, DAC runs at 44.1kHz when there is no valid digital signal NoDeEmphasis False, true (if true, disable de-emphasis support) Crossfeed_IgnoreHPSocket If true, crossfeed is not disabled when headphones not plugged in alongside the names, to make them easier to pick out. Settings memory Virtually all of the settings which can be changed with the remote control are “remembered” by the DAC when it is put into standby and even if it is powered off. This includes not only volume, tone control, crossfeed state etc but also which WAV file is being played, the position in the file and siliconchip.com.au so on. As long as you put the unit into standby before switching the power off, when you switch it back on, you will be right back where you were. The circuit incorporates some components so that if power is lost while the unit is running, it will attempt to save the current settings. However, the power supply capacitors collapse relatively quickly so it may not always have time to do so. Hence it’s always a good idea to go into standby before switching the unit off at the wall. Some settings can also be changed by putting a configuration file on the SD card (see below). These are loaded at power-on or when the card is inserted and if present, will override whatever the current settings are. Configuration file The configuration file is an optional text file placed in the root directory of the SD card, named “DAC.cfg”. It has one setting per line, with the setting name on the left followed by an equals sign (=) and then the value on the right. Tables 3 and 4 show all the possible settings. Settings not listed in the file will not be changed. Here is a sample configuration file: Auto_Play = true Init_File = \My Music\01 – First Track.wav Tone_Enabled = true Tone_AutoLoudness = true Crossfeed_Enabled = true Crossfeed_IgnoreHPSocket = false Bootloader Should you need to update the microcontroller firmware, eg, if a bugfix becomes available, this can be done from the SD card without the need for a programmer or even to open the case up. First, rename the new firmware file to DAC.HEX and place it the root directory of an SD card. Then, with the DAC off, insert the card in the socket and power the unit up. The sampling rate LEDs will start a chaser pattern while it checks the HEX file against the existing firmware. If it’s different, the unit will re-flash itself, using the eight input channel LEDs as a bargraph to indicate progress. Once programming is complete, the unit will boot into the new software and you can then remove the SD card and delete the file from it (it’s no longer needed). This “bootloader” function is particularly useful if you bought a pre-programmed chip and don’t have a PICkit to reprogram it later if new software is made available. That completes the description of how to operate the CLASSiC DAC. As is typical for our projects, the full software source code (around 9000 lines of C) is available for download from our website in a zip file. We will also supply a small document with an overview of the software and a brief explanation of how some of its SC parts work. May 2013  63