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AV (audio video) transmitter-receivers operating at 2.4GHz have been around for a while and have found many uses, especially in and around the home and in PA applications.We have often thought that their audio performance is not as good as we might hope. So we put a couple of typical units through their paces. Q: How good are those A: JUST AWFUL! By Allan Linton-Smith W e tested two different units, both of which operate at 2.4GHz. One came from Dick Smith Electronics (remember them?) – Cat A2288, which retailed for $179. The other one was purchased on ebay for $50 (including shipping) and it is a generic model, #PAT220, from a Fig.1: The Dick Smith unit has an unusual frequency response in the left and right channels and a somewhat flatter response for the mono AV input. 86  Silicon Chip Hong Kong supplier. Both units operate at a frequency of 2.4GHz although the DS model quotes 16QAM/QPSK/BPSK modulation modes to ensure security and interference-free operation. The generic model operated at a fixed frequency but other frequencies were selectable via a DIP switch arrangement Fig.2: This is the frequency response of the generic 2.4GHz unit and smoother in all modes than the DS unit. siliconchip.com.au 2.4GHz AV “Senders”? to “tune out” any local interference or to adjust for the best picture and sound. Both units were quoted as having a range of approximately 30 metres indoors and 100 metres outdoors and had a signal strength of around -23dBm at one metre. Both units included an attachment to transmit IR signals for distant remote control to operation of devices such as TV, DVD etc. According to the instruction booklet for the generic unit, the IR extender operates at 433.92MHz with a carrier frequency of 32kHz. We assume that the DS model operates in a similar fashion but they do not specify any frequencies and we did not test Fig.3: Separation between channels for the DS unit is satisfactory below 10kHz but is almost non-existent above that frequency. siliconchip.com.au the IR remote function of either unit, preferring to focus on the performance of the more important audio signals. Frequency response The response of the Dick Smith unit was quite flat, over the range from 200Hz to about 12kHz, and almost identical for the left and right audio channels, as shown in the cyan and magenta curves in Fig.1. The response of the generic unit was better, with -3dB points at 12Hz and 17kHz, as shown in Fig.2. There was a slight difference between left and right audio channels of around 0.5dB but this would undetectable in listening tests. Fig.4: Separation between channels for the generic 2.4GHz unit is satisfactory over the full audio bandwidth. JJuly uly 2016  87 2016  87 Fig.5: total harmonic distortion versus frequency for the DS 2.4GHz unit at 1V RMS. Note the rise to 9% at 10kHz and above. Separation between channels The Dick Smith unit claimed good performance on this parameter, with separation of better than -70dB for both channels (ie, the effect of a LH signal on the right channel and vice versa), as shown in Fig.3. But for frequencies above 10kHz, there was virtually no separation at all! That means that the resulting stereo “stage” will not be as wide and well-defined as you would get from a well-recorded CD. On the other hand, as shown in Fig.4, the generic unit was significantly better right across the range and channel separation was more than -60dB from 10Hz to 40kHz; not that anyone can hear frequencies above 20kHz! Harmonic distortion Now we know that some of our readers think that we have an obsession with vanishingly low distortion levels (well OK, we do; see the Ultra-LD Mk4 amplifier) but even Fig.6: Total harmonic distortion versus signal level for the DS 2.4GHz unit at 1kHz. At 2V RMS it is above 20%; excruciatingly bad! fairly average audio equipment should manage a harmonic distortion performance of better than 0.5% over the audio range at typical signal levels. The Dick Smith unit did not even come close. At an input signal level of 1V RMS (half the maximum signal level from a DVD or CD player) total harmonic distortion and noise (THD+N) was a whopping 5% at 1kHz and it climbed to 9% at 10-12kHz. See Fig.5. These measurements seemed unusually high so all the test connections were double checked to ensure there were no hum loops. We also used three different Audio Precision analysers to confirm the measurements were correct and not created by any instrument problems. It turned out that some of this distortion was created by good old-fashioned signal clipping, as can be seen from the waveforms in Scope1. But even reducing the signal level does not bring about a major improvement; tests were also carried out on THD+N The generic unit was bought on ebay for just $50.00 – including postage from Hong Kong! 88  Silicon Chip siliconchip.com.au Fig.7: The FFT spectrum analysis for the THD measurement in Fig.5 reinforces the story: heaps of high level harmonics. versus input strength and somewhat better figures (less bad!) were obtained at input levels of less than 0.5V RMS but this would still not be considered “hi-fi”. See Fig.6. And have a look at the THD when signal level rises to 2V RMS – the distortion is well above 20% – that’s due to severe clipping. Note that there is no input level control on the Dick Smith 2.4GHz AV unit or any other generic unit that we know of, so if you plug in a typical DVD or CD player which has a maximum audio signal output level of 2V RMS, you will inevitably get bad distortion. It cannot be avoided unless you can reduce the signal level. That’s hard to believe but true. Furthermore, if you look at the FFT analysis for the DS unit it shows a virtual forest of harmonics! In fact the third harmonic of 3kHz is only 27dB lower than the fundamental signal of 1KHz! See Fig.7. Results on the generic unit were actually worse; extremely high in the bass frequencies with a whopping 10% at 20Hz. See Fig.8. Fig.9 shows that performance is slightly improved at input levels of less than 1V RMS but increases significantly at levels lower than 200mV RMS. And have a look at the THD as the level goes above 1V RMS; it is almost the same as the DS unit; awful, The FFT analysis of the generic unit was better than the Dick Smith unit with a third harmonic 42 dB below the fundamental. See Fig.10. Overall, the distortion performance of both units was just poor. Fig.8: Total harmonic distortion versus frequency for the generic 2.4GHz unit at 1V RMS. Note the rise to 10% at 20Hz; not good. in direct line of sight. The DS unit paired in well, with the picture held solidly and there was absolutely no interference. The sound was steady with no pops or crackles but that is the good news. Otherwise, the sound quality can only be described as poor, with very noticeable distortion at the higher frequencies. Percussion instruments, for example, came across with a typical “crashing wave” distorted noise effect. This may not be a problem for normal domestic TV programs but it would definitely be unpleasant if you like to listen to music through a good quality amplifier and speakers or at high sound pressure levels. PA, guitar etc. The picture quality can best be described as average – there was a distinct degradation of picture quality (pixelation) which was very noticeable when the TV was switched between direct input and transmitted input – so much so that some subtitles were barely readable. The Dick Smith unit operates in the same 2.4GHz band. It also includes a set of 3-way A/V cables and an infrared receiver (not shown here). Transmitted bandwidth The OBW (occupied bandwidth) of the Dick Smith unit was 78.8MHz at a CF(centre frequency) of 2.4425GHz which is exactly within the manufacturer’s specification. The generic unit was factory set at 2.3726GHz but other settings could be made via the DIP switches up to 2.4537GHz; these had to be set on both the transmitter and receiver. Hence, the total usable bandwidth was around the same at 81MHz. Subjective testing Both units were tested on a Samsung 80cm TV set with normal speakers using a variety of program material. The transmitter was placed 10m from the receiver but was not siliconchip.com.au July 2016  89 Fig.9: Total harmonic distortion versus signal level for the generic 2.4GHz unit at 1kHz. At 2V RMS it is approaching 20%; also excruciatingly bad! Fig.10: The FFT spectrum analysis for the THD measurement in Fig.9 reinforces the story: not as bad as the DS unit but still awful. By contrast, the generic unit was significantly better in the high distortion and poor frequency response from the the picture department with no hint of pixelation and all transmitted signals. subtitles were clear and readable. Hard-wired extensions such as low resistance speaker The sound was not too bad either and you would have to wire or balanced line cable or twisted cable arrangements listen carefully for distortion when the sound was switched for hi-fi or PA would be a better way to go for long distances. between direct and transmitted. We did not use large speakDon’t forget that the ID security coded signals from the ers but we would expect distorted bass notes below 50Hz. DS unit ( but not the generic unit) will prevent anyone else But rather unfortunately, the generic unit suffered terribly from tuning in and this would be recommended for CCTV from both interference and pops and crackles from other security systems but not really necessary for movies or TV household modem and WiFi devices, phones etc. programs. This was hard, if not impossible to tune out using the With the single frequency generic unit, several receivers DIP switches and required some patience because, just can be tuned to one transmitter and this would be handy when everything seemed interference free, an unexpected for PA at sporting events where it would be cheap and easy crackling signal would pop up . to set up several powered speakers. Also the signal was interrupted altogether when a large Another advantage of the DIP switches is that they allow person (me) stood in the signal path. the user to “tune” in to various PA zones such as a dressing This would prove really annoying if you were watching room or a marshalling area a long movie or sports event. The RF interference which was obvious in the cheaper In summary, despite the generally poor audio tests, these generic unit could possibly be corrected with a fair bit of devices may be quite fiddling and setting the satisfactory in non-crittransmitters and receivical applications. ers above head height to If you want a quick, avoid signal loss. easy remote AV set-up The price of these that doesn’t require 2.4GHz AV links are high fidelity, they can coming down all the work quite well. time but we would genIf you need to transerally caution against mit TV to your bedusing them for music rooms or CCTV security and video if you require to remote locations or high quality sound and for “ordinary” PA, you vision. will enjoy the versatilOf course, we have ity of these units. only tested two of these But if you like listenunits. Others on the ing to music and are market might have using a high quality much better perforsignal source, a high mance. But unless the quality amplifier and vendors make specific speakers, or if you claims, you can probrequire them for PA ably assume that their Scope1: this scope grab shows the clipped signal at 1V RMS and the quality music – you resultant residual distortion waveform after the 1kHz fundamental performance is pretty SC will be disappointed at has been removed. A clipped signal will never sound good! mediocre. 90  Silicon Chip siliconchip.com.au