[Elecraft] external soundcards for digimodes

[Joe Subich, W4TV]

> 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
> 
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft    
> 
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
> 
>