[aprssig] A Closer Look at the WA8LMF TNC Test CD
Stephen H. Smith
wa8lmf2 at aol.com
Sat Jan 2 11:01:27 CST 2016
On 1/2/2016 2:04 AM, Rob Riggs via aprssig wrote:
> Hi John,
> Some of the distortion you show, especially the ones Dire Wolf shows the volume
> level over 100, look a lot like what one sees when overdriving the inputs on an
> audio capture device.
> How much of the distortion you are seeing is caused by the capture and
> recording process? How much is due to the radio used to capture the signal?
> Those seem to be unknowns and ought to be accounted for.
I made a serious effort to eliminate these kinds of issues when making the
I did the original capture with a Soundblaster PCI 128 card (the former
Ensoniq) and pointedly kept the peak level at -6 dB from the max possible in
Audition. During the subsequent editing, I used Audition to normalize the
audio level to within .5 dB of max; i.e each clip was scanned and adjusted so
that the loudest point in the clip was .5 dB down from the max possible. (This
is a routine process in digital audio editors.)
Since the un-squelched un-quieted receiver white noise between transmissions
was always higher than even fully-deviated signals, peak-level overload and
clipping would have only occurred on the noise, not the signals anyway.
The other issue would be the noise floor and distortion of the sound card
itself. I initially tested the card's performance by recording a single 1000 Hz
tone from one of my IFR-1500 service monitor's audio generators. I then played
the tone back through the playback side of the card into the IFR's SINAD meter.
In addition to the usual reverse-scale analog SINAD meter display that makes
it hard to read more than about a 20dB SINAD, the IFR can produce a digital
readout of SINAD on it's scope screen that will read out down to the noise
floor of the device at about 45 dB.
The result of the "round trip recording" was a 30+ dB SINAD, indicating that
the total noise and distortion added by a round trip through the sound card
card was less than 0.1%.
To verify the cleanliness of the audio source, I fed the 1K tone from the IFR's
audio generator directly into it's own SINAD meter input. The meter showed
only the residual noise floor of around 45 dB SINAD.
To eliminate the possibility of AC power-supply ground loops "contaminating"
the measurements, I used a 12 VDC battery to power the IFR using it's DC input
during the tests.
The radio used for the off-the-air tests was a Yaesu FT-1500 compact 2M FM
monobander tapped through it's 6-port mini-DIN "data port" using the "9600
baud" pin (i.e. the connection directly to the radio's raw discriminator
output). A test with the IFR's RF generator set to 50 microvolts and modulated
to 3.5K deviation with the 1K audio tone produced a 38 dB SINAD at the data
port pin. (And only about 21 dB at the speaker terminal, showing the
contribution of the less-than-Hf-Fi audio power amp!) The din-port SINAD went
down to about 21 dB when I advanced the generator deviation to a full 5 KHz.
Bottom line: Any distortion you are seeing is most likely due to either the
original on-the-air signal being distorted, or the playback CD player being
used having mediocre D/A converters or output line driver amps.
Stephen H. Smith wa8lmf (at) aol.com
EchoLink: Node # 14400 [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net
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