[aprssig] Interesting Preliminary Findings on Soundcard "Soft TNC" Shootout
Steve Noskowicz
noskosteve at yahoo.com
Mon Dec 26 10:35:18 EST 2011
Steve,
This is a well done test and report and expected based on what I know of your experience. One question is how many good packets in a row did you consider to be at the "consistently decoding the packet burst" point?
I'd say the 1 dB difference at 1200 baud is virtually insignificant, no?
I would have asked if you were planing pre/de-emphasis shaped tests, but you answered it already.
Not to wanting to make work for you, but an identical setup comparison with some of the Hardware TNCs would be really nice, I'm sure you'd have to admit.
--
73, Steve, K9DCI USN (Vet) MOT (Ret) Ham (Yet)
--- On Mon, 12/26/11, Stephen H. Smith <wa8lmf2 at aol.com> wrote:
> From: Stephen H. Smith <wa8lmf2 at aol.com>
> Subject: [aprssig] Interesting Preliminary Findings on Soundcard "Soft TNC" Shootout
> To: "TAPR APRS Mailing List" <aprssig at tapr.org>
> Date: Monday, December 26, 2011, 2:44 AM
> I have started comparison testing the
> relative performance of three sound-card software packet
> TNCs that can be used with APRS programs as a replacement
> for hardware TNCs such as a KPC3 or TNC2.
> The three "soft TNCs" I am testing are:
>
> o MixW 2.20 running in packet mode.
>
> o The most recent freeware version
> of the AGW Packet Engine (Ver 2010.414)
>
> o The new UZ7HO "soundmodem" beta
> Ver 0.41
>
> The "transmitter" in my test setup consists of the signal
> generator of my IFR-1500 communications
> monitor. Audio from a PC running UIview,
> the UZ7HO soundmodem and a Griffin Electronics "iMic"
> external USB sound system is fed into the external FM mod
> input of the IFR. This is a very
> low-distortion modulator with a flat response from less than
> 5 Hz to over 100 KHz. (I have actually fed the output
> of a homebrew stereo multiplex generator into this input
> with the IFR set to generate in the FM broadcast band,
> and have exceeded the performance of commercial FM stereo
> broadcast transmitters.) The RF output of the IFR is
> variable with a digital attenuator in 1dB steps from 0 dBm
> to -127 dBm.
>
> The receive setup for the first quickie tests was a Kenwood
> TM-G707 VHF/UHF FM transceiver. The IFR generator is
> connected to the antenna input of the Kenwood, as it would
> be for receiver alignment or a sensitivity/SINAD test. I am
> taking audio from the 6-pin mini-DIN "data"/"packet" jack,
> passing it through one of the homebrew soundcard interfaces
> described on my web site, and into the line-level audio
> input of a Dell L-866r PC. [The built-in sound
> system in this elderly 866 MHz Pentium III is not the
> typical low-performance brain-dead mostly-software-based
> AC'97 motherboard sound system found in virtually all
> current PCs. Rather there is an actual
> HARDWARE-based Soundblaster AudioPCI 64 built into the
> motherboard; i.e. the same chipset and accurate
> crystal-controlled timebase used on many PCI-card add-on
> sound systems.]
>
> The test consisted of repeatedly hitting "F9" in UIview to
> force a beacon transmission on demand, while gradually
> reducing the RF level of the generator in 1 dB steps, until
> the receive setup stopped consistently decoding the packet
> burst.
>
> I ran one of the softmodems described above at a time on
> the same PC, trying them at both the standard 1200
> baud 1200/2200 Hz tones and with 300 baud
> 2100/2300 Hz tones. The RF levels were measured
> in dBm rather than microvolts since the IFR's attenuator is
> calibrated this way. The correspondence,
> in 50-ohms systems, for those not familiar with dBm is:
>
> -119 dBm .252 uV
> -120 dBm .224 uV
> -121 dBm .200 uV
> -122 dBm .178 uV
> -123 dBm .159 uV
> -124 dBm .141 uV
> -125 dBm .126 uV
>
> At -119 dBm, the TM-G707 has definitely lost full quieting
> on an unmodulated carrier, with soft hiss evident. 1 dB
> lower at -120 dBm, "popcorn" type noise starts appearing ,
> along with the "fine-grained" hiss. (The
> transition from fine hiss to "popcorn" corresponds closely
> to the "12 dB SINAD" sensitivity in most narrowband FM
> receivers.)
>
> Note that the test setup has "flat" audio response on both
> transmit (no pre-emphasis in the IFR modulator), and on
> receive (no de-emphasis on the mini-DIN dataport output). As
> a result, more high-frequency hiss was present at low RF
> levels than a pre-empth/de-emph mic&speaker hookup would
> have.
>
> The results were:
>
> *** 1200 baud ***
> Lowest level (dBm) for reliable copy; i.e. EVERY
> packet decoded
> MixW 2.2
> -119 dBm
> AGWpe
>
> -119 dBm
> UZ7HO
> -120
> dBm
>
> *** 300 baud *** Lowest level (dBm) for reliable copy;i.e.
> EVERY packet decoded
> MixW 2.20
> -120
> AGWpe
>
> -121
> UZ7HO
>
> -123
>
> UZ7HO
>
> -124 (Still copying 90% of test packets!)
>
> 1) Note that these 300 baud tests were
> conducted over FM; not SSB. However, the 300 baud mode shows
> a worthwhile improvement in weak signal performance over
> 1200 even on FM. Conceivably this might be
> useful in some specialized low power application over a
> marginal path on FM. Note UZ7HO is
> fully-tunable to any audio tone pair at either baud
> rate. MixW is tunable at 300 baud but fixed at 1200,
> while AGWpe is fixed at both baud rates (1200/2200 at 1200
> baud and 2100/2300 at 300 baud).
>
> 2) The UZ7HO modem clearly outperforms the
> other two , especially at 300 baud.
>
> 3) In actual off-the-air testing on 30 meters
> HF APRS (i.e. SSB with lots of non-gaussian noise),
> with all three modems running at once off the same receiver
> (a Kenwood TS-690), I am seeing about 10 successful decodes
> for UZ7HO vs 7-8 for AGW vs about 1-2 for MixW.
> UZ7HO has the capability of using multiple additional
> pairs of audio detectors (a.k.a. "receivers") spaced in
> intervals of 30 Hz to each side of the main
> channel. These increase the CPU loading of
> the PC, but the UZ7HO modem then reliably and consistently
> copies off-frequency stations that the others completely
> miss, thus dramatically increasing it's edge over the other
> two.
>
> 4) I intend, in the next week or so, to expand
> this series of experiments to include a variety of radios,
> and to include some hardware TNCs, probably a KPC3+, a KAM
> and a PK-232.
>
> I will also experiment with mic jack/speaker audio hookups
> vs flat in/flat out. (The IFR has a mic
> jack with typical 3dB/octave pre-emphasis speech input - I
> need to make up a weird cable to go from a PC's 3.5 stereo
> minijack to the IFR's oddball-spaced 6-pin full-sized
> twist-lock DIN mic jack.)
>
> -------------------------------------------------------------------------------
>
> --
>
> Stephen H. Smith wa8lmf (at) aol.com
> Skype: WA8LMF
> Home Page: http://wa8lmf.net
>
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