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Question about pre-emphasis in SPDIF

Started by khung January 23, 2013
Hi everyone, 

I have a newbie question about pre-emphasis in SPDIF. I am referring to pp. 11 of IEC 60958-3. The standard has the following line: 

             2 audio channels with 50 microseconds / 15 microseconds pre-emphasis 

My understanding of de-emphasis is that a de-emphasis filter has one time constant and so in this case it should be either 50 microseconds or 15 microseconds. How do I know which time constant should I use in de-emphasis? 

Or in SPDIF, the de-emphasis is made up of 2 filters (one has 50 microseconds and another has 15 microseconds) connected in series? 

Any help or pointer would be great. 
Kevin 
"khung" <cykhung@gmail.com> wrote in message 
news:e2f89081-2600-4f05-bcfe-7eac79e7bb7d@googlegroups.com...
> Hi everyone, > > I have a newbie question about pre-emphasis in SPDIF. I am referring to > pp. 11 of IEC 60958-3. The standard has the following line: > > 2 audio channels with 50 microseconds / 15 microseconds > pre-emphasis > > My understanding of de-emphasis is that a de-emphasis filter has one time > constant and so in this case it should be either 50 microseconds or 15 > microseconds. How do I know which time constant should I use in > de-emphasis? > > Or in SPDIF, the de-emphasis is made up of 2 filters (one has 50 > microseconds and another has 15 microseconds) connected in series?
1 + s T1 H(s) = ----------- 1 + s T2 Vladimir Vassilevsky DSP and Mixed Signal Consultant www.abvolt.com
khung <cykhung@gmail.com> wrote:
 
> I have a newbie question about pre-emphasis in SPDIF. > I am referring to pp. 11 of IEC 60958-3. The standard has > the following line:
> 2 audio channels with 50 microseconds / 15 microseconds pre-emphasis
Pre-emphasis is optional, and relatively rare, on CDs, and yes, according to some web sites, when used it has the 50us/15us time constants. Seems to me that when SPDIF is used with CD data, the preemphais bit should come from the subcode data on the CD track. Are you assuming that all sources have it? -- glen
On 1/23/13 8:36 PM, glen herrmannsfeldt wrote:
> khung<cykhung@gmail.com> wrote: > >> I have a newbie question about pre-emphasis in SPDIF. >> I am referring to pp. 11 of IEC 60958-3. The standard has >> the following line: > >> 2 audio channels with 50 microseconds / 15 microseconds pre-emphasis > > Pre-emphasis is optional, and relatively rare, on CDs, > and yes, according to some web sites, when used it has the 50us/15us > time constants. > > Seems to me that when SPDIF is used with CD data, the preemphais bit > should come from the subcode data on the CD track. > > Are you assuming that all sources have it? >
my understanding was that S/PDIF was the same as AES-3 (sometimes called AES/EBU) but with different channel status bits and maybe different electrical specs and connectors. i remember hooking up a consumer digital output into an Eventide DSP-4000 (the former was S/PDIF and the latter AES) and it worked. i think it was because the receiver was ignoring channel status, i dunno. anyway, i had no idea that S/PDIF had pre-emphasis or an option for it. what's the point? to de-emphasize the quantization noise of the 16-bit words? -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
Back when the spec was originally written, there were no true 16 bit conver=
ters, so they applied the pre-emphasis/de-emphasis concept from the world o=
f analog tape recorders to improve the dynamic range. Of course at the time=
 they were envisioning an analog de-emphasis network with 2 resistors and a=
 capacitor and an analog switch. Now of course we do it digitally.=20

The annoying thing for chip designers like myself is that you need to put a=
 lot of effort into designing the de-emphasis filter just to cover the case=
 where someone plays back that 1-in-a-million CD from 1982. The reason it's=
 hard is that you need to dynamically change the filter coefficients for ev=
ery sample-rate you want to support. Makes you want to time-travel back to =
1980 and yell at the founding fathers.=20


Bob