Jerry Avins <jya@ieee.org> wrote:>Matt Timmermans wrote:>> These days, PCM is the name given to the common uncompressed >> representations of audio signals as bit sequences. Yes, I know >> that's silly, but that's the way it is.It's not silly at all; once you put the data through a compression algorithm, it no longer consists of distinct codes that represent the level of the analog signal at distinct sample instants. So it's no longer PCM.>If there's general agreement on the issue (there hardly seems to be), >that's fine with me, but nobody told me about it. Can you cite any >credible sources?I gave a definition of PCM upthread; an equivalent textbook definition is in Stremler, _Introduction to Communications Systems_ and many other texts. There should not be much confusion on this. Steve
What does PCM mean to you?
Started by ●October 3, 2005
Reply by ●October 4, 20052005-10-04
Reply by ●October 4, 20052005-10-04
Steve Pope wrote:> Jerry Avins <jya@ieee.org> wrote: > > >>Matt Timmermans wrote: > > >>>These days, PCM is the name given to the common uncompressed >>>representations of audio signals as bit sequences. Yes, I know >>>that's silly, but that's the way it is. > > > It's not silly at all; once you put the data through a > compression algorithm, it no longer consists of distinct codes > that represent the level of the analog signal at distinct > sample instants. So it's no longer PCM. > > >>If there's general agreement on the issue (there hardly seems to be), >>that's fine with me, but nobody told me about it. Can you cite any >>credible sources? > > > I gave a definition of PCM upthread; an equivalent textbook > definition is in Stremler, _Introduction to Communications Systems_ > and many other texts. There should not be much confusion on this.Are uncompressed data transmitted with more than one bit per symbol -- high-speed modem traffic, for instance -- PCM or not? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 4, 20052005-10-04
Reply by ●October 4, 20052005-10-04
Jerry Avins <jya@ieee.org> wrote:>Are uncompressed data transmitted with more than one bit per symbol -- >high-speed modem traffic, for instance -- PCM or not?I'm not sure I understand the question. Any analog signal passed into an A/D creates a PCM version of that analog signal, and the A/D can be more than one bit wide. Steve
Reply by ●October 4, 20052005-10-04
Steve Pope wrote:> Jerry Avins <jya@ieee.org> wrote: > > >>Are uncompressed data transmitted with more than one bit per symbol -- >>high-speed modem traffic, for instance -- PCM or not? > > > I'm not sure I understand the question. Any analog signal > passed into an A/D creates a PCM version of that analog > signal, and the A/D can be more than one bit wide.That's just a pulse code. There's no modulation. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 4, 20052005-10-04
"Jerry Avins" wrote ...> Are uncompressed data transmitted with more than one bit per symbol -- > high-speed modem traffic, for instance -- PCM or not?Doesn't quadrature modulation allow several bits per baud, something around 4-5?
Reply by ●October 4, 20052005-10-04
Jerry Avins <jya@ieee.org> wrote:> Steve Pope wrote:>> I'm not sure I understand the question. Any analog signal >> passed into an A/D creates a PCM version of that analog >> signal, and the A/D can be more than one bit wide.>That's just a pulse code. There's no modulation.I can understand why that is what you may think, but my statement above describes what is considered "pulse code modulation". It's just a definition, it's arbitrary, but it's used consistently in this way. Steve
Reply by ●October 4, 20052005-10-04
Jon Harris wrote:> "Jerry Avins" <jya@ieee.org> wrote in message > news:sMydnckE6OIQFN_eRVn-jg@rcn.net... > >>Matt Timmermans wrote: >> >>> These days, PCM is the name given to the common uncompressed >>> representations of audio signals as bit sequences. Yes, I know >>> that's silly, but that's the way it is. >> >> If there's general agreement on the issue (there hardly seems to >> be), that's fine with me, but nobody told me about it. Can you cite >> any credible sources? (Credible is a step down from >> authoritative.) There was a time when many took "communist" to mean >> any individual who disagreed with the speaker's politics, but that >> didn't make it so. > > I'm in agreement with Matt regarding the common use of PCM, at least > in the consumer audio industry. The term "PCM" in exactly the sense > Matt describes is used many times my home DVD player's documentation > (JVC XV-SA600). Some examples:I don't think the examples are using it in the "common use" sense you mean. Let's look at them one by one:> "There are some audio formats recorded on discs as shown below. > Linear PCM Uncompressed digital audio, the same format used on CDs > and most studio masters."Here it's qualified as "Linear ... Uncompressed", to give it the "common use" sense you expect.> "DVD with 192/176.4/96/88.2 kHz, 16/22/24 bit linear PCM"Here again "16/22/24 bit linear ...".> In fact, the digital output is labeled "PCM/Stream" (stream is used > to refer to Dolby Digital and other compressed/encoded formats).I.e., the general term "Pulse Code Modulation", or "Stream" (of bits) is used to cover any encoded sequence of bits, not just linear. PCM originally referred to the transmission of data samples as a stream of pulses chosen from a code set, as opposed to PWM (Pulse Width Modulation), where the width of each pulse would be proportional to the amplitude of the data sample, or PAM (Pulse Amplitude Modulation), where each pulse would be proportional to the amplitude of the data sample. The code set wouldn't need to be "zeros and ones", but could be something more complex, like "one, zero", "one, one", and so on (where each "zero" or "one" actually represents a pulse shape). It's all pretty logical. Paulo
Reply by ●October 4, 20052005-10-04
Jerry Avins wrote:> PCM stands for pulse code modulation. Can there be PCM without > modulation? Is it still PCM if the modulation carries more than one bit > per symbol?What a term or abbreviation, such as PCM, might mean is often domain dependent. Various computer, consumer electronics, and multimedia "standards" groups have often (re)defined a term when used inside their particular portion of an industry differently from the usage in other fields. So what I say below is probably completely irrelevant to consumer audio labeling. I go back to the days when AM, FM, PWM, PAM, PPM and PCM all referred to various methods of transferring information. PCM signals were differentiated from P?M in that the data was somehow encoded (via some table or time sequence usually, but not always, consisting of "ones" and "zeros"), and thus not directly related to the duration, amplitude, position or other analog demodulated characteristic of the signal. Unlike another poster, I don't think an A/D was necessary for a signal to be PCM. The data to be transfered could have started out completely in the digital domain (from a teletype keyboard for instance), thus requiring no conversion from analog. Since the information to be transfered was "coded" it could pretty much represent anything (encrypted streams, ASCII or EBCDIC, for instance. Maybe even morse code?) And once the information is stored, it doesn't seem to have anything to do with "pulses" or "modulation", just coding. IMHO. YMMV. -- rhn A.T nicholson d.O.t C-o-M
Reply by ●October 4, 20052005-10-04
On Tue, 04 Oct 2005 17:41:34 -0700, Paulo Castello da Costa wrote:> Jon Harris wrote: >> "Jerry Avins" <jya@ieee.org> wrote in message >> news:sMydnckE6OIQFN_eRVn-jg@rcn.net... >> >>>Matt Timmermans wrote: >>> >>>> These days, PCM is the name given to the common uncompressed >>>> representations of audio signals as bit sequences. Yes, I know that's >>>> silly, but that's the way it is. >>> >>> If there's general agreement on the issue (there hardly seems to be), >>> that's fine with me, but nobody told me about it. Can you cite any >>> credible sources? (Credible is a step down from authoritative.) There >>> was a time when many took "communist" to mean any individual who >>> disagreed with the speaker's politics, but that didn't make it so. >> >> I'm in agreement with Matt regarding the common use of PCM, at least in >> the consumer audio industry. The term "PCM" in exactly the sense Matt >> describes is used many times my home DVD player's documentation (JVC >> XV-SA600). Some examples: > > I don't think the examples are using it in the "common use" sense you > mean. Let's look at them one by one: > >> "There are some audio formats recorded on discs as shown below. Linear >> PCM Uncompressed digital audio, the same format used on CDs and most >> studio masters." > > Here it's qualified as "Linear ... Uncompressed", to give it the "common > use" sense you expect. > >> "DVD with 192/176.4/96/88.2 kHz, 16/22/24 bit linear PCM" > > Here again "16/22/24 bit linear ...". > >> In fact, the digital output is labeled "PCM/Stream" (stream is used to >> refer to Dolby Digital and other compressed/encoded formats). > > I.e., the general term "Pulse Code Modulation", or "Stream" (of bits) is > used to cover any encoded sequence of bits, not just linear. > > PCM originally referred to the transmission of data samples as a stream > of pulses chosen from a code set, as opposed to PWM (Pulse Width > Modulation), where the width of each pulse would be proportional to the > amplitude of the data sample, or PAM (Pulse Amplitude Modulation), where > each pulse would be proportional to the amplitude of the data sample. > The code set wouldn't need to be "zeros and ones", but could be > something more complex, like "one, zero", "one, one", and so on (where > each "zero" or "one" actually represents a pulse shape). It's all pretty > logical.I agree. To elaborate: The important point about "PCM" (any of the forms) is that it is fully wide-band. There is one independent code word per sample period. (As is the case with PWM and PAM, of course.) ADPCM is also one code per sample, but the code is dependent on the preceding samples, and so typically has frequency-dependent limitations. DD/AC3, DTS, MP3 etc are much more complicated encodings, generally with much more complicated frequency-dependent limitations, and generally not one code-word per sample period. Well, that's my understanding. -- Andrew
