[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 
recordings.

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
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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