Dolby Digital/Dolby Surround

shinobi

Can any one tell me what the difference is between Dolby Digital & Dolby Surround is, it seems to vary in the programme synopsis from movies to movie.

ozpass

Dolby surround (or it's more evolved relations Pro Logic and Pro Logic II) are analogue surround technologies. They analyse the stereo spread of the signal and provide portions of the stereo soundstage to additional rear speakers, creating the 'surround effect'. Dolby Digital is a digital technology (funnily enough) and provides distinct signals to distinct channels (Left, Centre, Right, Right Rear, Left Rear, Subwoofer in a 5.1 setup). The 5.1 in the name corresponds to (1,2,3,4,5, .1) channels of audio in the example I've given.

This is explained very succintly (and better than me) on several sites on the internet. A quick google just turned up http://www.virtualmags.com.au/Sofa/articles/basic_surround_sound.htm which seems to explain it reasonably well (I didn't read more than the first paragraph.......)

watty

Dolby Surround:

Just matrixing 2 rear channels onto L and R. Dolby wasn't first. There are a few variations of it. It is relatively easy to create or decode with analog Op Amps. Simplest "decode" is to connect a pair of rear speakers in Series BETWEEN the Live Left out and Live Right Out of the Amplifier.
You can code it (or Decode it) better in DSP on a Digitised signal. Does not require a digital Source or Digital Decoder, though both can be used.

I think there are 3 or 4 kinds of Dolby Digital ..
Pro Logic
Dolby 5.1
Are two common kinds.

AFIK the Pro Logic is a "better" form of Surround using something more than simple matrix. The "Surround" can seem a bit artifical and creates "false" rear sound and also is limited on separation. Pro Logic solves this using a stereo transmission (2 channels) but a digital decoder.

Dolby 5.1 is very fake surround sound generated on a mixing desk for Theatrical Effects. The 5th center cjhannel is artifical since even with Any real stereo this can be generated simply by adding R+L. The .1 is a very limeted bandwidth "rumble" effects channel. It has two separate L & R rear channels.

Look up "ambisonic" for info about "realistic" surround sound. "Quadraphonic" was simply an early analog attempt at at what "Pro Logic" does with a digital decoder.

Look up "Binaural" on Google too to read about true surround sound recordings for headphones. Ordinary Stereo or Dolby is for loudspeakers it doesn't work properly psycho-acoustically for headphones. Binaural does. Sennhisser does a "box" that creates quasi-binaural signal from ordinary stereo to greatly enhance headphone listening (ordinary stereo sounds like it is inside your head).

ProLogic and Dolby 5.1 are fine for Cinematic content. They are very unrealistic for Concert or live performace sound, esp. for Folk, Jazz and Classical.

Most Pop music stereo is poor as each source is "pan potted" between Left and Right channel on final mix varying only in amplitude. The sources should have varying absolute delays and L<->R relative delays / phase change to simulate a real sound field with electronic or close "miked" sources. You can as long ago as mid 70s get mixing desks that do this. Most don't, only varying the "loudness" as you pan source left/right.

a) You can record a live performace with two cardiod mikes crossed at 120 degrees horizontally (capsules ovelap closely vertically) about 6ft to 40 ft from stage (depending if it is "folk" acoustic instruments, Chamber music, Orchestra or Heavy Metal), about 4ft off ground.

b)At same time record each instrument / artist with a connection or close up microphone, mixing down to stereo using pan pots.

Recording (a) is true Stereo and will even give signifcant rear speaker echo/ reverberation matching orignal hall on a Dolby Surround decoder with 4 speakers. The front artist locations will be sharp and clearly defined. It may work even better with a prologic decoder! And we used no coder, just one mike into Left and one into right.

Recording (b) is very artifical with no rear channel sound at all. The locations of sounds will be indistinct. Recording (B) can be done properly but only if you add absolute delays and relative L/R delays to simulate path to a hypothetical listening position. Also the reveberation and acustics of the hall are totally missing as if it was recored in a "dead" studio or a quiet hill top.

watty

http://www.virtualmags.com.au/Sofa/...round_sound.htm
Is of course grossly inaccurate as many people had surround sound before Dolby even existed as Company (originally a Noise Reduction system for analog audio tape was sole product).

Getting Stereo or Surround with "Pictures" at home in 60s and 70s was a challenge! The BBC occasionally broadcast FM Stereo Radio (which you could feed into your 1960s/1970s equivalent of Dolby Surround) a simulcast of the TV program as TV did not have Stereo. Opera, Classical Music and ocassionally even Rock was broadcast this way.

This why many VHS VCRs still have feature to record picture from Tuner and sound via L & R phono/RCA sockets from separte FM Radio.

They also broadcast some Binaural in early 1970s too.

I built a home made surround sound system with 4 ch Valve amplifer (Ex WD 6V6 valves and scrap TV parts) and four matching TV speakers in homemade cabinets around early 1970s. (A typical Mid 1960s TV speaker is twice size of the little speakers on so called home cinema systems). By mid 1970s I had a transistor amplifier. My own TV in my Flat was still B&W though it did have 625 lines with the 405 line changeover disabled.

Most PC stereo speakers are poor, and they are often better than the 4 satellite speakers on many "Home Cinema" systems.

ShaneOC

Originally posted by shinobi
Can any one tell me what the difference is between Dolby Digital & Dolby Surround is, it seems to vary in the programme synopsis from movies to movie.

Some (most) of the movies on Sky Premiere Widescreen are broadcast in Dolby Digital 5.1. However to take advantage of this you need to have the Sky+ receiver (not available here yet) connected to a dolby digital amplifier.

Krusty_Clown

Shinobi: You can of course watch (listen) to some Sky Digital programs in Dolby Pro-logic surround sound, if you have the necessary equipment..

I read somewhere that SKY+ may soon be supporting (and broadcasting) in DTS.. Wahoo! Roll-on SKY+.. (please!)

watty

It costs more to transmit Dobly 5.1, it uses a separate data stream to normal audio PID.
Scanning nearly 750 FTA channels (and some susbscription), I didn't find ONE with 5.1 Audio stream. It seems to be only some of the PPV.

So even with a Sky+, on a Family pack you won't get Dolby 5.1.

ShaneOC

Originally posted by watty

So even with a Sky+, on a Family pack you won't get Dolby 5.1.

Yes it is only available on Sky Premiere Widescreen and Sky Box Office Widescreen, and even then not on 100% of the movies.