[Elecraft] K3 and USB Audio Codec

[Kok Chen]

On Jul 10, 2010, at 7/10    2:57 PM, Joe Subich, W4TV wrote:

> If one were going to provide digital output, it would be far better
> to stream the raw in I/Q data from the DSP's ADC via firewire rather
> than push it through two additional A/D and D/A steps and reduce it
> to a single channel of audio.

FireWire is not necessarily a great solution either.  It will  
definitely require special drivers at the computer end (viz, the  
Edirol FA-66 that is common in ham SDR circles).

One good connection that I have come across is S/PDIF that is  
available on the Icom IC-7800.  It allows a connection between the  
radio and the computer that is completely non-galvanic.   No ground  
loops, no RFI, down to -144 dB.

S/PDIF provides an audio path with up to 20 bits of resolution (about  
120 dB, with 1 bit being 6.02 dB), with 24 bits being an option by  
using the extra 4 steering bits as data.

The standard sampling rate for S/PDIF is 48,000 samples per second  
stereo, which would support almost 50 kHz of bandwidth on an baseband  
I/Q channel.  S/PDIF provides up to about 30 feet separation between  
equipment.

Both Firewire and S/PDIF lacks a standard way of passing control/ 
status signals. The Flex-5000 (which uses FireWire) used to hack into  
the MIDI (Musical Instrument Digital Interface) channel for control/ 
status; but I have not been following that development and it might  
have since changed.  The Icom IC-7800 goes through its regular CAT  
path (RS-232 or CI-V, you get to choose, I believe).

In any case, if you want a rig like the K3 to support 123 dB of  
dynamic range I/Q output, it will not come cheap.  The Asahi AK-5394A  
is probably the most expensive component in the front end of the  
Flex-5000, and you have to be very careful with the circuit layout to  
get a -125 dB noise floor when mixing analog and digital components.

We are definitely not talking about 16-bit codecs like the one in the  
SignaLink USB -- which by the way, is a TI/Burr-Brown PCM-2902, and it  
is not the $20 that people have been mentioning, but is $5.85 at  
DigiKey :-).

73
Chen, W7AY