Joe,
I agree with most all of it except the end. That's a discussion
many have been involved in for the last 10 years that I know of.
You are right on when you say a 16 bit A/D will work fine for the
times you are taking the line out out for the demoded audio. I seriously
doubt that many communications receivers' audio have the dynamic range
of the A/D following.
I do use the raw I and Q out of my KX3. It's one of the best
selling features of the radio, besides its inherent good performance.
Interestingly enough, I pipe the output of my 24 bit card which is used
for basic demod to my 16 bit card for digital work. And, why not. I
control the level going into the latter card directly so that it doesn't
overload. In other words, I might be viewed as cheating. But, it does
work great. I do highly recommend some form of this to anyone who wants
to try something that is fun and effective. I do admit it had it
frustrations, but the result has been very pleasing. My next step is to
get a cable that I can feed audio back into the mic connector and
control the PTT.
73,
Barry
K3NDM
On 8/21/2012 2:28 PM, Joe Subich, W4TV wrote:
> On 8/21/2012 11:18 AM, Jim Brown wrote:
> > On 8/21/2012 7:48 AM, Barry LaZar wrote:
>>> However, 24 bits of quantization will get you a better dynamic
>>> range.
> >
>> Not if the dynamic range of the analog electronics limits it, which
>> is what Joe has pointed out. Often those extra bits are "advertising
>> bits" -- they fool those who don't know any better into believing
>> that more bits is better.
> There are two issues here -
>
> First most amateurs run their digital software from "line out" or the
> demodulated audio from the transceiver. The demodulated audio has
> generally been through at least one A to D to A cycle and the noise
> floor is simply too high to take advantage of even the theoretical 96
> dB dynamic range of a 16 bit sound card.
>
> Now, if one is looking at the "raw" I/Q RF/IF or direct downconversion
> audio 24 bits "might" be usable *IF* the noise floor is low enough (a
> real *IF* with off the air signals) and any mixers and amplifiers are
> really linear through the theoretical 138 dB dynamic range *without* AGC.
>
> None of this addresses the noise floor and dynamic range of any preamp
> in the sound card ... it only looks at the theoretical dynamic range
> of the analog to digital converter.
>
> Then there is an additional consideration pointed out by Chen in a
> private e-mail - properly designed software can achieve dynamic range
> (sensitivity) *better than* the A to D converter (that is, it can
> "receive into the noise") by applying proper lowpass filtering and
> decimation. I know Chen uses decimation even with 16 bit cards in
> cocoaModem but it does not appear that any of the Windows centric
> digital software does so since many are based on 11025 or 12000 Hz
> sample rates and effectively throw away the potential processing gain
> by not performing any lowpass filtering in the 44100/48000 to 11025
> or 12000 sample rate downconversion.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 8/21/2012 11:18 AM, Jim Brown wrote:
>> On 8/21/2012 7:48 AM, Barry LaZar wrote:
>>> However, 24 bits of
>>> quantization will get you a better dynamic range.
>> Not if the dynamic range of the analog electronics limits it, which is
>> what Joe has pointed out. Often those extra bits are "advertising bits"
>> -- they fool those who don't know any better into believing that more
>> bits is better. Actually, it's far more important to optimize the
>> signal level that the A/D converter sees. That's why I like the Tascam
>> USB interface better than the Numark -- it has a much greater range of
>> gain adjustment and a clip light on the front panel. That allows me to
>> easily optimize the 90 dB or so of dynamic range that its 16 bits provides.
>>
>> Besides -- the 90 dB dynamic range provided by a decent 16-bit A/D is
>> probably more than most hams will ever see at the audio output of their
>> radios. While the radio itself has much greater dynamic range, the
>> WORKING dynamic range is limited by noise received by the antenna that
>> is often 30-50 dB greater than the electrical noise levels inside the
>> radio.
>>
>> 73, Jim K9YC
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