In a local emergency 'net we were talking about the effect of fire fronts on vhf/uhf communications. The Royal Commission in Victoria published this article "Effect of Fire Fronts on V/UHF Radio Communications":
- http://38c69b050a3d5d1eb1e3-aa923a4231e15c57e2802c896554e8a6.r6.cf4.rackcdn.com/6/6AD7B8F1-2ECF-4AFB-8208-EE1C6AB2607E.PDF On 24 July 2015 at 11:39, Erich Heinzle <[email protected]> wrote: > It seems the emergency responders in the US Fire Services looked at > background noise and P25 radio codec performance for their safety critical > workers soon after its introduction. > > http://www.iafc.org/associations/4685/files/digProj_DPWGinterimReport.pdf > http://www.its.bldrdoc.gov/publications/08-453.aspx > > http://utahvhfs.org/dstar_codec_behavior.html > > FWIW, the Utah site mentions how companding may confound encoding of low > level audio in the presence of background noise. > > Regards, > > Erich. > > > >> Message: 3 >> Date: Fri, 24 Jul 2015 09:33:15 +0930 >> From: David Rowe <[email protected]> >> Subject: Re: [Freetel-codec2] microphones >> To: [email protected] >> Message-ID: <[email protected]> >> Content-Type: text/plain; charset=windows-1252; format=flowed >> >> Thanks Jean-Marc, >> >> Just had a chat with some local Hams on our morning "drive time" net and >> I have a radio receiver model: >> >> + a microphone is an antenna with a noise figure. We assume linearity >> for now (the microphone isn't clipping and is linear). >> >> + To remove some variables lets say we take extacly the same mic and try >> it 2cm and 50cm away from the speakers lips. >> >> + If you take the same microphone and move it away the signal power >> drops and (given the same noise figure) SNR must decrease. Adding extra >> gain doesn't help the SNR, just like adding gain down the track in a >> radio receiver doesn't help the SNR. >> >> + Over a longer path, different frequency components will be attenuated >> by different amounts, as the lips are not equally efficient in radiating >> at all frequencies. Here we diverge from the radio analogy; radio >> signals are usually narrow compared to the antenna bandwidth. >> >> + A microphone further away picks up multi-path reflections from the >> room, laptop case, that start to become significant compared to the >> direct path. Summing a delayed version of the original signal will have >> an impact on the frequency response and/or add reverb - just like a HF >> or VHF radio signal. These may be really hard to remove. >> >> -/- >> >> Re the Speex denoiser, yes good idea. I have that in command line unit >> test form in codec2-dev (so I can pipe audio files through it) and BTW >> we are also using it in the FreeDV GUI program where it's helping with >> background noise - thanks Jean Marc :-) >> >> -/- >> >> Gd point on the energy of the low freq speech components - I am looking >> into a related problem (see Glen's post) with samples that have much >> greater LF than HF energy. These tend to do poorly, which I think is >> due to the LPC (short term spectrum) model struggling with the high >> dynamic range. LPC analysis designs a filter to mimimise the energy in >> the residual (the signal after filtering), so will "throw poles" at the >> high energy parts first. >> >> Our ears, however have a log response to energy, so the codec breaks and >> sounds bad (or rather even worse Lol). >> >> Thanks, >> >> David >> >> On 24/07/15 08:16, Jean-Marc Valin wrote: >> > Hi David, >> > >> >> > > ------------------------------------------------------------------------------ > > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > -- Onno Benschop ()/)/)() ..ASCII for Onno.. |>>? ..EBCDIC for Onno.. --- -. -. --- ..Morse for Onno.. If you need to know: "What computer should I buy?" http://goo.gl/spsb66 ITmaze - ABN: 56 178 057 063 - ph: 04 1219 8888 - [email protected]
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