yeah you can pretty much get away with NO aliasing filter considering the source ( a band limited microphone) and the required voiceband SN.
still, good reason to have at least a single RC- from say switchmode power supply whistles etc getting back into the input, and aliasing down into the baseband (from say 50-300kHz) , or even the transmitter. Yes-on 160m, I have had the RF aliasing back into the passband from the 32kHz sampled mic audio..... g On 18/09/2015 7:35 PM, David Rowe wrote: > Also there is very little speech energy out past 12 kHz, i.e. its > already 50dB down, and this is communications quality speech so a little > aliasing is lost in the codec artefacts. > > On the radios interface side the audio is band limited by the radio's > frequency response, ie the xtal filter BW. > > - David > > On 18/09/15 15:43, 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 >>> Freetel-codec2@lists.sourceforge.net >>> 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 >> Freetel-codec2@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > ------------------------------------------------------------------------------ > _______________________________________________ > Freetel-codec2 mailing list > Freetel-codec2@lists.sourceforge.net > 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 Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
