Subject: Re: RE : - "Birdies" From: Max Neuendorf <max.neuendorf@xxxxxxxx> Date: Fri, 17 Mar 2006 18:16:06 +0100Hi Maxime, hi everyone, The term "Birdies" is indeed (also) used for audiocoding artifacts, which occur when reducing the bitrate to very low figures. As has been pointed out by other here on this list, the coding algorithm then starts removing single frequency components altogether or dynamically changes the audio bandwidth, which can both result in the same kind of "bird singing" or "ringing" sound degradation. You might want to have a look at a tutorial CD published by the AES Technical Committee at the 111th Convention in New York in September 2001. Its called "Perceptual Audio Coders “What to listen for” " and comes with a nice paper by Markus Erne, which explains some of the observed phenomena. You can find the CD here: http://www.aes.org/publications/AudioCoding.cfm Best regards, Max Maxime Leroy wrote: > Thank you Bob for you comments, > > "But simple early schemes had interactions between the input signal > and the sample frequency that caused "birdies" at sum and/or > difference frequencies." > > I realise now "birdies" might not be exactly what i meant. I will > rephrase then, if you ever looked closely at the spectrogram of a > sample of music encoded at low bit-rate (20-64Kbps)by either mp3 or > AAC codecs, you might have noticed dark spots in some places where it > is obvious the energy of the signal is not suppose to be so small. I > suppose that artifact is due to the richness of the signal at this > precise moment (in comparison with the bit-rate) and therefore bit > allocation can not cope with the demand. Then the coder being unable > to encode leaves a hole in the spectrogram. > > If i'm correct with the above assumption, what i'd like to know is if > there is any documentation or perceptual intepretation of this > problem of coding. > > Regards, > > Maxime > > > ________________________________ > > De: AUDITORY Research in Auditory Perception de la part de Bob Masta > Date: mer. 15/03/2006 14:30 À: AUDITORY@xxxxxxxx Objet : Re: - > "Birdies" > > > > Hi, Maxime. I'm not sure exactly what you are looking for, and I > don't have any references to provide. But if you are looking for a > perceptual description, here's what I know: > > "Birdies" are little whistling sounds that are related to the program > material, but are not harmonics of it. They used to be a serious > problem in sigma-delta converters, which compare the input signal to > a reconstruction of the output signal, and generate a "higher than" > or "lower than" response on each sample. That 1-bit stream is then > used to create the reconstruction for the comparison (and the > eventual output). Nowadays, this is all done at very high sample > rates and then ultimately converted down to a nominal rate, and the > reconstruction processing is very sophisticated. But simple early > schemes had interactions between the input signal and the sample > frequency that caused "birdies" at sum and/or difference frequencies. > The birdies might be only 40 dB down, but even if they were much > softer than that they were clearly audible, especially on sparse > program material like simple sine waves, flutes, etc, since they > appeared in non-harmonic locations and were not masked by the program > itself. They also often had the annoying habit of sweeping in the > opposite direction to a sweep in the signal frequency, which made > them really obvious. > > > Hope that helps! > > Best regards, > > Bob Masta > > > -- Max Neuendorf Audio Coding & Multimedia Software Fraunhofer IIS Am Wolfsmantel 33 91058 Erlangen, Germany Phone: +49 (0)9131 / 776-329 Fax: +49 (0)9131 / 776-398 max.neuendorf@xxxxxxxx www.iis.fraunhofer.de Meet us at these events: CeBIT Hannover, 9. - 15. March DVB World Dublin, 1. - 3. March VON San Jose, 14. - 17. March ...