[Elecraft] external soundcards for digimodes

[Joe Subich, W4TV]

> The SSM2165 is a good speech compressor and not expensive.  
> I'm assuming it will work on data....?  to bring up the level 
> as required.

You don't want compression on data modes.  Better to use a good 
low noise, single rail op amp with a low noise reference as a 
preamplifier to bring the maximum receive level up to 4 V P-P. 
Compression effects the characteristics of some data modulation 
modes (one reason for the suggestions to not run any transmit 
ALC with data modes other than FSK). 

73, 

   ... Joe, W4TV 
 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of David Cutter
> Sent: Thursday, August 16, 2007 1:19 PM
> To: Joe Subich, W4TV; 'Michael Keane K1MK'; 'Elecraft Reflector'
> Subject: Re: [Elecraft] external soundcards for digimodes
> 
> 
> The SSM2165 is a good speech compressor and not expensive.  
> I'm assuming it 
> will work on data....?  to bring up the level as required.
> 
> You can get a complete kit from www.box73.com DYC-817   (Code 734980)
> External dynamic compressor kit for FT-817.
> 
> Or from OK1CDJ, but I can't find the link at the moment.
> 
> This will incur some extra noise, but there are components to 
> adjust to 
> optimise compression etc.
> 
> David
> G3UNA
> 
> 
> ----- Original Message ----- 
> From: "Joe Subich, W4TV" <w4tv at subich.com>
> To: "'Michael Keane K1MK'" <k1mk at alum.mit.edu>; "'Elecraft 
> Reflector'" 
> <elecraft at mailman.qth.net>
> Sent: Thursday, August 16, 2007 5:50 PM
> Subject: RE: [Elecraft] external soundcards for digimodes
> 
> 
> >
> >> Obviously, the performance specs for a 24-bit sound card should be
> >> better than for a 16-bit card.
> >>
> >> Does anyone know which software apps actually do make use of the
> >> extra bits and the higher performance of a 24-bit sound card?
> >
> > There is not much that can be done in the application to take
> > advantage of the 24 bit sound cards.  All of the devices are
> > limited on the "strong signal" end of the spectrum by their power
> > supply (typically 5 volt) which limits the signal to the ADC to
> > about 4.5 volts p-p.  On the low signal end, the digital software
> > is limited by the noise level from a "quiet band" (receiver IF
> > noise, atmospheric noise, etc.) at the receiver output.  
> From memory,
> > most receivers I've seen show composite noise floors about 80 dB
> > below their peak output (peak output is generally around 200 mV
> > average or somewhat less than 1 volt p-p) with no antenna and
> > "quiet band" noise around 70 dB below maximum output.
> >
> > Even if the DSP algorithms in the digital software could get 10
> > dB below the noise, the 16 bit sound cards with their 95 to 97
> > dB theoretical dynamic range would be more than enough for most
> > amateur work at HF.
> >
> > They key to getting everything you can with soundcard modes on
> > HF is having a very low noise audio amplifier between the
> > receiver and ADC (it may be in the soundcard) and minimizing
> > any power supply and logic noise in the sound card.  If you
> > will be using AGC and narrow filters in the receiver (one
> > key to maximum weak signal effectiveness), the receiver AGC will
> > limit the dynamic range into the sound card to well less than
> > 40 dB (the difference between MDS and AGC threshold) in any case.
> >
> > You want the peak audio to be as close to the 4 V p-p point
> > as possible to make maximum use of the sound card's available
> > dynamic range as you can.  If the maximum signal to the ADC is
> > only one volt p-p you have already given away 12 dB of DR!
> >
> > 73,
> >
> >   ... Joe, W4TV
> >
> >
> >
> >> -----Original Message-----
> >> From: elecraft-bounces at mailman.qth.net
> >> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of
> >> Michael Keane K1MK
> >> Sent: Thursday, August 16, 2007 11:49 AM
> >> To: Elecraft Reflector
> >> Subject: Re: [Elecraft] external soundcards for digimodes
> >>
> >>
> >> At 09:08 AM 8/16/2007, Don Wilhelm wrote:
> >> >Typically, the more bits in the Soundcard A-D conversion, 
> the better
> >> >the dynamic range.  There is also the noise floor of the
> >> soundcard to consider.
> >> >
> >> >Most internal soundcards are 16 bit while the upgrades 
> are typically
> >> >24 bits for the A-D.
> >>
> >> Obviously, the performance specs for a 24-bit sound card should be
> >> better than for a 16-bit card.
> >>
> >> However, in his review of sound cards for Amateur Radio 
> that appeared
> >> in the May 2007 QST, Jonathan Taylor, K1RFD wrote that very few
> >> digital-mode apps are designed to take advantage of the higher
> >> precision when it is available.
> >>
> >> Does anyone know which software apps actually do make use of the
> >> extra bits and the higher performance of a 24-bit sound card?
> >>
> >> 73,
> >> Mike K1MK
> >>
> >> Michael Keane K1MK
> >> k1mk at alum.mit.edu
> >>
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