Couldn't you do further low-pass filtering in software, then decimate to 8 kHz? (caveat: I haven't checked if the code actually does this)
/Tomas On Fri, 2015-09-18 at 16:13 +1000, glen english wrote: > Hi Stuart > > An elaboration on David's reply. > > We want our audio BW to go to at least say 3.4 kHz . That is what > fixed line telephones offer. > > If we were to sample at 8kHz, our nyquist frequency is 4kHz of course. > Any spectral information above the nyquist rate will be aliased bay > into the baseband, IE below the nyquist rate. > > IF we assume we don't care about aliases between 3.4 and 4kHz, then > this allows for signals up to 4.0 - 3.4 = 0.6, and 0.6+Fyquist(4kHz) > = 4.6kHz. (as a 4.5kHz signal will get aliased to 3.5 kHz.) > > Assume we want our aliases 50dB down, so our aliasing filter must go > from 0dB down at 3.4kHz to 50 dB down at 4.6kHz > > Clearly alot of work in the analog department- probably needs an > elliptic filter. > a transition bandwidth factor of (4.6/3.4) = 1.35 > > Now, if the sample rate was 16kHz, now, Fnyquist is 8 kHz. again, we > dont care about aliases above 3.4kHz, > and Fnyquist, now 8kHz - 3.4kHz is 4.6kHz. > > So, for freq up to Fnyquist + 4.6kHz - IE 12.6kHz will not alias into > problem areas. > > so, now our analog filter spec if relaxed > 0dB down at 3.4kHz, and 50dB down at 12.6kHz > a transition bandwidth factor of (12.6/3.4) = 3.7 > > > > QED > > > > > On 17/09/2015 7:53 PM, Stuart Longland wrote: > > > Hi all, > > > > Just looking at the source code, it hit me. When in analogue mode, we > > run 16kHz sample rate, fair enough. > > > > But in DV mode, we sample the ADC at 16kHz, downconvert to 8kHz, pass it > > through the modem and codec, then upconvert back to 16kHz for the DAC. > > > > We're dealing with voice frequencies, with SSB transmit bandwidths of > > less than 3kHz. > > > > Why not do the whole lot at 8kHz and save some CPU time? Maybe some > > rate-switching when in analogue or DV mode might be an option too, so we > > run ADC/DAC at full-rate in analogue (for higher fidelity), then switch > > to half-rate when we're doing DV. > > > > Regards, > > > > > > ------------------------------------------------------------------------------ > > Monitor Your Dynamic Infrastructure at Any Scale With Datadog! > > Get real-time metrics from all of your servers, apps and tools > > in one place. > > SourceForge users - Click here to start your Free Trial of Datadog now! > > http://pubads.g.doubleclick.net/gampad/clk?id=241902991&iu=/4140 > > > > > > _______________________________________________ > > Freetel-codec2 mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > -- > - > Glen English > RF Communications and Electronics Engineer > > CORTEX RF > & > Pacific Media Technologies Pty Ltd > > ABN 40 075 532 008 > > PO Box 5231 Lyneham ACT 2602, Australia. > au mobile : +61 (0)418 975077 > > ------------------------------------------------------------------------------ > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2
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