Dolby Pro Logic

I've just upgraded from a non-HDMI AV amp to an HDMI one, the main input
is my mythTV PC via HDMI (still using audio over SPDIF temporarily) ...
it's nice to have switching on the TV automatically power up the amp
too, and the TV's remote control the amp's volume and input selection.

I have the amp's surround mode set to "auto".

When watching a normal programme (i.e. not a film) the amp correctly
selects Stereo.

When watching something with 5.1 audio (e.g. a film from a freesat HD
channel) the amp automatically selects Dolby Digital and is in full 5.1 mode

When watching a film (from a freesat or freeview SD channel) it never
seems to automatically choose Pro Logic, if I force the setting, the
decoding seems to work, I can hear the 'steering' around the channels.
If the encoding is there, why can't the amp detect it and automatically
enable it?

Or am I fooling myself on material that doesn't have any encoding, and
I'm just hearing a synthetic surround effect?

How much freeview/freesat material does have Pro Logic encoding?

Andy Burns wrote:
When watching a film (from a freesat or freeview SD channel) it never
seems to automatically choose Pro Logic, if I force the setting, the
decoding seems to work, I can hear the 'steering' around the channels.
If the encoding is there, why can't the amp detect it and automatically
enable it?

Maybe I'm missing something here, but I don't think that any Freesat SD
channels carry Dolby Digital audio.

John Legon wrote:

Maybe I'm missing something here, but I don't think that any Freesat SD
channels carry Dolby Digital audio.

No, I'm not expecting them to have Dolby Digital, but do they carry
Dolby Pro Logic encoded in their stereo?

I thought (at least some) films did, should a decoder normally switch
automatically if the encoding was there?

John Legon wrote:
Andy Burns wrote:
When watching a film (from a freesat or freeview SD channel) it never
seems to automatically choose Pro Logic, if I force the setting, the
decoding seems to work, I can hear the 'steering' around the channels.
If the encoding is there, why can't the amp detect it and
automatically enable it?

Maybe I'm missing something here, but I don't think that any Freesat SD
channels carry Dolby Digital audio.

No not 5.1 audio, but Pro Logic audio should make its way through the system,
as long as the MPEG coding is transparent enough. In reality I think if the
audio is anything less than 256kb/s it won't work ?

--
Mark
Please replace invalid and invalid with gmx and net to reply.

www.paras.org.uk

"Andy Burns" wrote in message
o.uk...

Or am I fooling myself on material that doesn't have any encoding, and
I'm just hearing a synthetic surround effect?

How much freeview/freesat material does have Pro Logic encoding?

My AV amp is handling all stereo in PL II mode and at times BBC1 HD or BBC
HD have a DD sound it changes automatically to DD.
My understanding of PL decoding is that it takes the mono content of the L
and R stereo and puts it through the centre speaker.
IF the program does have surround, such as a movie might have, that will go
to the rear speakers.

I would suggest you set your amp to PL and see if on BBC HD versions see if
some programs change automatically to DD as mine does.

Regards
David

In article ,
Andy Burns wrote:

I've just upgraded from a non-HDMI AV amp to an HDMI one, the main input
is my mythTV PC via HDMI (still using audio over SPDIF temporarily) ...
it's nice to have switching on the TV automatically power up the amp
too, and the TV's remote control the amp's volume and input selection.

I have the amp's surround mode set to "auto".

When watching a normal programme (i.e. not a film) the amp correctly
selects Stereo.

When watching something with 5.1 audio (e.g. a film from a freesat HD
channel) the amp automatically selects Dolby Digital and is in full 5.1 mode

When watching a film (from a freesat or freeview SD channel) it never
seems to automatically choose Pro Logic, if I force the setting, the
decoding seems to work, I can hear the 'steering' around the channels.
If the encoding is there, why can't the amp detect it and automatically
enable it?

Or am I fooling myself on material that doesn't have any encoding, and
I'm just hearing a synthetic surround effect?

How much freeview/freesat material does have Pro Logic encoding?

Dolby Stereo soundtracks are technically speaking just two-channel
stereo. There is no way of identifying from the soundtrack the fact that
it is in Dolby Stereo, and Freeview doesn't carry any flag to identify
this.

In Dolby Stereo, the centre channel is reduced in level by 3dB and
placed equally in left and right. The single rear channel is
phase-reversed, Dolby B encoded, and added to the left and right.
(Therefore if you listen in mono the rear channel disappears). On
playback a logic steering system recognizes the four channels and
sort-of separates them (there is a good deal of leakage). At its very
simplest:

Sound in L only appears in the left channel.
Sound in R only appears in the right channel.
Sound which is the same in L & R is placed in the centre channel and
muted in the left and right channels.
Sound which is the same in L & R but out of phase is placed in the rear
channel and muted in the other three.

Modern decoders can be quite sophisticated in steering the sound to
provide a convincing soundstage, though they can't position discrete
sounds in the rear in the way that Dolby Digital can.

You will always have to switch Dolby Digital decoding on manually.

Mark Carver wrote:
John Legon wrote:
Andy Burns wrote:
When watching a film (from a freesat or freeview SD channel) it never
seems to automatically choose Pro Logic, if I force the setting, the
decoding seems to work, I can hear the 'steering' around the
channels. If the encoding is there, why can't the amp detect it and
automatically enable it?

Maybe I'm missing something here, but I don't think that any Freesat
SD channels carry Dolby Digital audio.

No not 5.1 audio, but Pro Logic audio should make its way through the
system, as long as the MPEG coding is transparent enough. In reality I
think if the audio is anything less than 256kb/s it won't work ?

If that's the case, then you might hope to get surround sound with
movies shown on BBC 1, 2 & 4 but not on ITV 1 or Channel 4 & 5 ... SD,
based on current bit rates.

Roger Wilmut wrote:

Dolby Stereo soundtracks are technically speaking just two-channel
stereo. [] the centre channel is reduced in level by 3dB and placed
equally in left and right. The single rear channel is phase-reversed,
Dolby B encoded, and added to the left and right. [] On playback a logic
steering system recognizes the four channels and sort-of separates
them

Thanks Roger, I was aware that it's just a stereo signal, rather than 6
discrete channels, and that it used phasing/matrixing to do the L/R/C/S
steering.

You will always have to switch Dolby Digital decoding on manually.

I presume you mean Pro Logic rather than Digital there? Seems a shame
there's no way to automatically detect it, but thanks for confirming.

"Roger Wilmut" wrote in message
...



You will always have to switch Dolby Digital decoding on manually.

Seems to be only at times and on HD from the BBC as far as I have
experienced, so how would a person who's equipment that is not automatic
know to do it manually?
Regards
David

In article ,
Andy Burns wrote:
No, I'm not expecting them to have Dolby Digital, but do they carry
Dolby Pro Logic encoded in their stereo?

Any stereo signal can carry pro logic - that's the point of it. But how
much is made in that these days I don't know.

--
*Who are these kids and why are they calling me Mom?

Dave Plowman London SW
To e-mail, change noise into sound.