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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.
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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.
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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!
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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
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