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, > > > >
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