Subject: RE=?iso-8859-1?Q?=A0:?= - "Birdies" From: Maxime Leroy <m.leroy@xxxxxxxx> Date: Wed, 15 Mar 2006 19:34:21 -0000This is a multi-part message in MIME format. ------_=_NextPart_001_01C64867.882418C4 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Thank you Bob for you comments, =20 "But simple early schemes had interactions between the input signal and the sample frequency that caused "birdies" at sum and/or difference frequencies." =20 I realise now "birdies" might not be exactly what i meant.=20 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. =20 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. =20 Regards, =20 Maxime =20 ________________________________ De: AUDITORY Research in Auditory Perception de la part de Bob Masta Date: mer. 15/03/2006 14:30 =C0: 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 ------_=_NextPart_001_01C64867.882418C4 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable <META HTTP-EQUIV=3D"Content-Type" CONTENT=3D"text/html; = charset=3Diso-8859-1">=0A= =0A= <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">=0A= <HTML>=0A= <HEAD>=0A= =0A= <META NAME=3D"Generator" CONTENT=3D"MS Exchange Server version = 6.5.7233.28">=0A= <TITLE>Re: - "Birdies"</TITLE>=0A= </HEAD>=0A= <BODY>=0A= <DIV id=3DidOWAReplyText25025 dir=3Dltr>=0A= <DIV dir=3Dltr><FONT face=3DArial color=3D#000000 size=3D2>Thank you Bob = for you =0A= comments,</FONT></DIV></DIV>=0A= <DIV>=0A= <DIV id=3DidOWAReplyText92379 dir=3Dltr>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2></FONT> </DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2><EM>"But simple early schemes = had =0A= interactions<BR>between the input signal and the sample frequency that =0A= caused<BR>"birdies" at sum and/or difference = frequencies."</EM></FONT></DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2></FONT> </DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2>I realise now "birdies" might = not be =0A= exactly what i meant. </FONT></DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2>I will rephrase then, if you = ever looked =0A= closely at the spectrogram of a sample of music encoded at low =0A= bit-rate (20-64Kbps)by either mp3 or AAC codecs, you might = have =0A= noticed dark spots in some places where it is obvious the energy of = the =0A= signal is not suppose to be so small. I suppose that artifact is = due to the =0A= richness of the signal at this precise moment (in comparison with = the =0A= bit-rate) and therefore bit allocation can not cope with the demand. = Then the =0A= coder being unable to encode leaves a hole in the = spectrogram.</FONT></DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2></FONT> </DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2>If i'm correct with the above = assumption, =0A= what i'd like to know is if there is any documentation or = perceptual =0A= intepretation of this problem of coding.</FONT></DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2></FONT> </DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2>Regards,</FONT></DIV>=0A= <DIV dir=3Dltr><FONT face=3DArial size=3D2></FONT> </DIV></DIV>=0A= <DIV dir=3Dltr>Maxime</DIV>=0A= <DIV dir=3Dltr><BR> </DIV>=0A= <DIV dir=3Dltr>=0A= <HR tabIndex=3D-1>=0A= </DIV>=0A= <DIV dir=3Dltr><FONT face=3DTahoma size=3D2><B>De:</B> AUDITORY Research = in Auditory =0A= Perception de la part de Bob Masta<BR><B>Date:</B> mer. 15/03/2006 =0A= 14:30<BR><B>=C0:</B> AUDITORY@xxxxxxxx<BR><B>Objet :</B> Re: - =0A= "Birdies"<BR></FONT><BR></DIV>=0A= <DIV>=0A= <P><FONT size=3D2>Hi, Maxime. I'm not sure exactly what you are = looking for, =0A= and I don't<BR>have any references to provide. But if you are = looking for =0A= a perceptual<BR>description, here's what I know:<BR><BR>"Birdies" are = little =0A= whistling sounds that are related to the<BR>program material, but are = not =0A= harmonics of it. They used to<BR>be a serious problem in = sigma-delta =0A= converters, which compare the<BR>input signal to a reconstruction of the = output =0A= signal, and generate<BR>a "higher than" or "lower than" response on each =0A= sample. That<BR>1-bit stream is then used to create the = reconstruction for =0A= the<BR>comparison (and the eventual output). Nowadays, this is = all<BR>done =0A= at very high sample rates and then ultimately converted<BR>down to a = nominal =0A= rate, and the reconstruction processing is<BR>very sophisticated. = But =0A= simple early schemes had interactions<BR>between the input signal and = the sample =0A= frequency that caused<BR>"birdies" at sum and/or difference = frequencies. =0A= The birdies might<BR>be only 40 dB down, but even if they were much = softer than =0A= that<BR>they were clearly audible, especially on sparse program = material<BR>like =0A= simple sine waves, flutes, etc, since they appeared in<BR>non-harmonic = locations =0A= and were not masked by the program<BR>itself. They also often had = the =0A= annoying habit of sweeping in the<BR>opposite direction to a sweep in = the signal =0A= frequency, which made<BR>them really obvious.<BR><BR><BR>Hope that =0A= helps!<BR><BR>Best regards,<BR><BR>Bob =0A= Masta<BR></FONT></P></DIV></DIV>=0A= =0A= </BODY>=0A= </HTML> ------_=_NextPart_001_01C64867.882418C4--