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[linrad] Re: 40 kHz of bandwidth with NO extra PCI-cards; adapting output sampling (CPU-load); optimum frequencies; Mini-ITX



Historically speaking, this is one-half of a common technique for upsampling: emit the sample followed by N zeros. The missing half of the technique is an IIR filter processing that output. Unfortunately this saves you essentially nothing since the proper IIR filter is high-order. It's mostly useful when the data to be upsampled have been pre-computed and the filtering is the only processing to be done.

I think I may have some code written by Ken Stieglitz lying around here, that computes the coefficients of the optimal elliptic filter for this task. I can dig it up if anybody's interested. There are better methods, however; it's basically obsolete at this point.

73
Frank
AB2KT

Joe Taylor wrote:
'Zaba' OH1ZAA wrote:

... Since most boards and integrated audio have coupled sampling
rates (48000/48000), would it be possible to prepare the D/A-output
bit-streams with modifications that could alleviate the workload of
the CPU? More specifically: would it be feasible to replace output
samples with strings of plain zeros thus mimicking 6000 and 8000 Hz
output rates? Example: output rate 48000 Hz, target 8000 Hz >>>>
calculate Lagrange-interpolation, feed to D/A-converter, fill next
5 samples with plain "0", then calculate the new D/A-output value.
Thus effective output rate will fall to 8000 Hz. Possibly the only
side effect with regular audio would be that the output level will
drop accordingly (and maybe an extra output filter is needed).


Audio produced this way would sound *terrible*. Much better would be to compute proper values at, say, an 8 kHz rate and output each value 6 times to make up the 48 kHz sample rate. With a 3 kHz outboard low-pass filter, I think that should sound pretty good.

		-- 73, Joe, K1JT

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