A Basic Question or two on CD Emphasis/Deemphasis

Started by September 16, 2004
What is it? 
Why is it used? 
How is it usually implemented (digitally or analogically)?
-- 
Randy Yates
Sony Ericsson Mobile Communications
Research Triangle Park, NC, USA
randy.yates@sonyericsson.com, 919-472-1124
On 16 Sep 2004 09:53:37 -0400, Randy Yates
<randy.yates@sonyericsson.com> wrote:

>What is it?
High frequency boost, defined by a single continuous time zero. IIRC the time constant is 15us.
>Why is it used?
Is it used? I don't think I have any CDs in my collection that use preemph. I guess it seemed like a good idea at the time. (Remember that the red book CD format was thought up in the 1970s, with 1x oversampling and analog reconstruction filters in mind. The extra lowpass pole in the player helps.)
>How is it usually implemented (digitally or analogically)?
Switchable (in or out) analog filter if > ~10 years ago, digitally (in the DAC) in contemporary players, digitally (in SW) in player software on your desktop computer. Regards, Allan
Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> writes:

> On 16 Sep 2004 09:53:37 -0400, Randy Yates > <randy.yates@sonyericsson.com> wrote: > > >What is it? > > High frequency boost, defined by a single continuous time zero. > IIRC the time constant is 15us.
OK, thanks Allan. Is the high frequency boost done on the signal at CD record time, then the inverse (high frequency attenuate) done at CD playback time? Or is it vice-versa?
> >Why is it used? > > Is it used? I don't think I have any CDs in my collection that use > preemph.
<Argh!> It's IN the DAMN PLAYER SO IT MUST BE USED! At least there was the intention for it to be used. Most designers I know of don't design stuff in for no reason. "Hey, let's add a waffle iron to this GlidePath navigation radio...".
> I guess it seemed like a good idea at the time. (Remember that the > red book CD format was thought up in the 1970s, with 1x oversampling > and analog reconstruction filters in mind.
> The extra lowpass pole in > the player helps.)
That could be the reason. Is it?
> >How is it usually implemented (digitally or analogically)? > > Switchable (in or out) analog filter if > ~10 years ago, digitally (in > the DAC) in contemporary players, digitally (in SW) in player software > on your desktop computer.
Not to look a gift whore in the mouse, but how do you know all this? -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Randy Yates <randy.yates@sonyericsson.com> writes:
> [...] > Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> writes: > > > I guess it seemed like a good idea at the time. (Remember that the > > red book CD format was thought up in the 1970s, with 1x oversampling > > and analog reconstruction filters in mind. > > > The extra lowpass pole in > > the player helps.) > > That could be the reason. Is it?
1. If this process is done in the analog domain at both ends, then what you gain in the reconstruction filter is lost in the anti-aliasing filter. 2. Why couldn't this have been done independent of the medium? That is, if the *player* DIGITALLY emphasized the signal, then the analog reconstruction filter, which would include the deemphasis filter, would have a better time of things, and the CD proper need not be aware of any of this. 3. Why isn't this done even today? Even a delta sigma D/A could use some help in the analog filtering stage. I have been hearing about CD emphasis/deemphasis since they came out around 1983 and have never heard a satisfying, authoritative answer as to why it is used. Frustrating! -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Randy Yates wrote:
> What is it?
Emphasis / De-Emphasis a complementeray cut-and-boost pair of high-shelve filters used before the ADC stage and after the DAC stage (ie. the producer of the CD applies emphasis to the analogue signal, and the CD player then has to apply de-emphasis). The transition band is specified by the 50/15 rule (50us and 15us timing constants for the corner frequencies), the gain is about 10 dB (specs should be available on the net).
> Why is it used?
If you assume that high frequency content of music is low in average (decreases with -6dB/oct for example), then boosting the high frequencies would increase the SNR in that range without affecting the average SNR in the low frequencies too much.
> How is it usually implemented (digitally or analogically)?
Obviously, it should be implemented in the analogue domain. However, if you convert an AES/EBU digital audio stream to some other format (for example raw data, as Rune likes to :-), you should theoretically check the emphasis bit, and apply a digital de-emphasis if necessary. I once converted the analogue filter specs to digital filter (sorry, don't have the files handy, as I'm not at work). There has been plenty of talk on how to do this best here in comp.dsp, it is slightly non-trivial because bi-linear transform introduces too much warping. However, I don't think there are many CDs that actually use emphasis, so it's a pretty academic subject. Regards, Andor
Randy Yates wrote:

> Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> writes: > > >>On 16 Sep 2004 09:53:37 -0400, Randy Yates >><randy.yates@sonyericsson.com> wrote: >> >> >>>What is it? >> >>High frequency boost, defined by a single continuous time zero. >>IIRC the time constant is 15us. > > > OK, thanks Allan. Is the high frequency boost done on the signal > at CD record time, then the inverse (high frequency attenuate) done > at CD playback time? Or is it vice-versa? > > >>>Why is it used? >> >>Is it used? I don't think I have any CDs in my collection that use >>preemph. > > > <Argh!> It's IN the DAMN PLAYER SO IT MUST BE USED! At least there was > the intention for it to be used. Most designers I know of don't design > stuff in for no reason. "Hey, let's add a waffle iron to this GlidePath > navigation radio...".
My car has some bulbs on the dash that I hope never light up, a spare tire that I hope never to use, and other "conveniences" that I'm probably unaware of. As for optional deemphasis in a player, maybe it's simply part of an old spec.
>>I guess it seemed like a good idea at the time. (Remember that the >>red book CD format was thought up in the 1970s, with 1x oversampling >>and analog reconstruction filters in mind. > > >>The extra lowpass pole in >>the player helps.) > > > That could be the reason. Is it? > > >>>How is it usually implemented (digitally or analogically)? >> >>Switchable (in or out) analog filter if > ~10 years ago, digitally (in >>the DAC) in contemporary players, digitally (in SW) in player software >>on your desktop computer. > > > Not to look a gift whore in the mouse, but how do you know all this?
Hey! it's Allan Herriman you're asking! That's as close to a dumb question as I've seen here in a long time. He's our resident encyclopedia for audio and broadcast specifications. ;^) Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Jerry Avins <jya@ieee.org> writes:
> [...] > > Not to look a gift whore in the mouse, but how do you know all this? > > > Hey! it's Allan Herriman you're asking! That's as close to a dumb > question as I've seen here in a long time. He's our resident > encyclopedia for audio and broadcast specifications. ;^)
I wasn't aware of this title for him. How STUPID of me! (Or at least ignorant...) -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
On 2004-09-16 16:55:23 +0200, Allan Herriman 
<allan.herriman.hates.spam@ctam.com.au.invalid> said:

> On 16 Sep 2004 09:53:37 -0400, Randy Yates > <randy.yates@sonyericsson.com> wrote: > >> What is it? > > High frequency boost, defined by a single continuous time zero. > IIRC the time constant is 15us.
From what I remember there was a 15us and a 50us setting. But this was a *long* time ago... back in the 80s I think... :-)
>> Why is it used? > > Is it used? I don't think I have any CDs in my collection that use > preemph.
Same here. I don't think it was ever really used. That idea may have come from the older vinyl gear, where pre-emphasis was widely used. -- Stephan M. Bernsee http://www.dspdimension.com
On 16 Sep 2004 11:21:02 -0400, Randy Yates
<randy.yates@sonyericsson.com> wrote:

>Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> writes: > >> On 16 Sep 2004 09:53:37 -0400, Randy Yates >> <randy.yates@sonyericsson.com> wrote: >> >> >What is it? >> >> High frequency boost, defined by a single continuous time zero. >> IIRC the time constant is 15us. > >OK, thanks Allan. Is the high frequency boost done on the signal >at CD record time, then the inverse (high frequency attenuate) done >at CD playback time?
Yes.
>> >Why is it used? >> >> Is it used? I don't think I have any CDs in my collection that use >> preemph. > ><Argh!> It's IN the DAMN PLAYER SO IT MUST BE USED! At least there was >the intention for it to be used. Most designers I know of don't design >stuff in for no reason. "Hey, let's add a waffle iron to this GlidePath >navigation radio...".
Most designers follow the standard, if there is one. The Red Book says to support optional deemphasis in the player, so they do. Whether it gets used is another matter. There's a flag in the format somewhere that indicates that whether preemp has been used. I don't know whether it's a per disk or per track thing. As I said, most disks are recorded without preemphasis. I think your question should have been "why did they put preemphasis in the Red Book?" The answer to that can be found in any undergrad communications course. The audio signal being recorded tends to have less power at high frequencies. The digitising process tends to make more noise at high frequencies. So boost the highs during recording and cut them on playback to improve the overall SNR. Note that (US) FM radio (1940s?) uses preemphasis with a 75us time constant. Many other parts of the world (which got FM radio much later than the US) use 50us. CD (1970s) is switchable to 15us or none. I believe this trend (of smaller time constants = less boost) over time is related to the improvement in recording techniques, microphones, etc. that causes audio to have more high frequency content. (BTW, 16 bit DACs weren't available until the mid '80s, and the early players used 14 bit DACs with a simple PWM arrangement (no noise shaping, etc.) so any extra LP filtering after the DAC was welcome.)
>> >How is it usually implemented (digitally or analogically)? >> >> Switchable (in or out) analog filter if > ~10 years ago, digitally (in >> the DAC) in contemporary players, digitally (in SW) in player software >> on your desktop computer.
Note that deemphasis *before* the DAC (either done digitally in the DAC or in software) doesn't reduce the noise of the DAC at all, which makes the whole scheme pointless. The deemphasis filter is still needed to get the overall frequency response flat, of course.
>Not to look a gift whore in the mouse, but how do you know all this?
I just made it up. A few data points plus lots of interpolation can work wonders. I've repaired plenty of CD players in my time, so I know the old ones have analog deemph. I read plenty of chip data sheets, so I know that some DACs (destined for CD players) have built-in digital deemph. Soundcard DACs don't have that, therefore the deemph, if needed, is done in software. Regards, Allan
Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> writes:

> On 16 Sep 2004 11:21:02 -0400, Randy Yates > <randy.yates@sonyericsson.com> wrote: > > >Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> writes: > > > >> On 16 Sep 2004 09:53:37 -0400, Randy Yates > >> <randy.yates@sonyericsson.com> wrote: > >> > >> >What is it? > >> > >> High frequency boost, defined by a single continuous time zero. > >> IIRC the time constant is 15us. > > > >OK, thanks Allan. Is the high frequency boost done on the signal > >at CD record time, then the inverse (high frequency attenuate) done > >at CD playback time? > > Yes.
OK, cool.
> >> >Why is it used? > >> > >> Is it used? I don't think I have any CDs in my collection that use > >> preemph. > > > ><Argh!> It's IN the DAMN PLAYER SO IT MUST BE USED! At least there was > >the intention for it to be used. Most designers I know of don't design > >stuff in for no reason. "Hey, let's add a waffle iron to this GlidePath > >navigation radio...". > > Most designers follow the standard, if there is one. The Red Book > says to support optional deemphasis in the player, so they do. > Whether it gets used is another matter. > > There's a flag in the format somewhere that indicates that whether > preemp has been used. I don't know whether it's a per disk or per > track thing. As I said, most disks are recorded without preemphasis. > > I think your question should have been "why did they put preemphasis > in the Red Book?"
And how is that question different than: "Why is it used?" ????????????????????????????????????????
> The answer to that can be found in any undergrad communications > course.
> The audio signal being recorded tends to have less power at high > frequencies.
Why?
> The digitising process tends to make more noise at high > frequencies.
Why would you say that? The standard assumption that quantization noise is white says there is not more noise at high frequencies.
> So boost the highs during recording and cut them on > playback to improve the overall SNR.
Oh, yeah - I should have known THAT. How *STUPID* of me!!!!! EXCEPT I didn't "know" that a) the audio signal has less power at high frequencies and b) quantization noise has more power at high frequencies. :) And what happened to your reconstruction filter conjecture???
> Note that (US) FM radio (1940s?) uses preemphasis with a 75us time > constant. Many other parts of the world (which got FM radio much > later than the US) use 50us. CD (1970s) is switchable to 15us or > none. > I believe this trend (of smaller time constants = less boost) over > time is related to the improvement in recording techniques, > microphones, etc. that causes audio to have more high frequency > content.
Hmmm.
> (BTW, 16 bit DACs weren't available until the mid '80s, and the early > players used 14 bit DACs with a simple PWM arrangement (no noise > shaping, etc.) so any extra LP filtering after the DAC was welcome.)
Why would DAC resolution (at least for a "standard" DAC like these, what, R2R ladder types) have any bearing on the filtering that was required?
> >> >How is it usually implemented (digitally or analogically)? > >> > >> Switchable (in or out) analog filter if > ~10 years ago, digitally (in > >> the DAC) in contemporary players, digitally (in SW) in player software > >> on your desktop computer. > > Note that deemphasis *before* the DAC (either done digitally in the > DAC or in software) doesn't reduce the noise of the DAC at all, which > makes the whole scheme pointless. The deemphasis filter is still > needed to get the overall frequency response flat, of course. > > >Not to look a gift whore in the mouse, but how do you know all this? > > I just made it up. A few data points plus lots of interpolation can > work wonders.
Ha! Thanks for the banter, Allan!
> I've repaired plenty of CD players in my time, so I know the old ones > have analog deemph. I read plenty of chip data sheets, so I know that > some DACs (destined for CD players) have built-in digital deemph. > Soundcard DACs don't have that, therefore the deemph, if needed, is > done in software.
Good info. Thanks! -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124