Home Cinema switching between LPCM and Dolby Digital
I Have a Sony BDV E670W Home Cinema System connected to a Sony 3D TV Via HDMI cable. I also have the home cinema system connected to a SKY HD box via digital optical cable to get 5.1. The TV is also connected to a PS3 via HDMI cable if it makes any difference.
When watching Sky the display on the home cinema says LPCM. In this format you can alter any sound options. Occasionally it also displays Dolby Digital and the sound is far better than LPCM, but it won't let you alter the sound options - although it defaults to Pro Logic sometimes. After changing channels the home cinema returns to the LPCM format but if I was to disconnect and reconnect the HDMI cable into the home cinema it will return to the Dolby Digital format.
How do I keep the home cinema in the Dolby Digital format?
Your question makes sense - up until the last part about unplugging the HDMI cable!
When you watch any type of broadcast TV, the audio can be transmitted in a couple of different formats. The signal that you receive is determined by the broadcaster.
These days, you will receive a mixture of stereo and multichannel sound (also known as surround sound or 5.1 audio). Most TV shows are still mainly in stereo audio - while a few will have 5.1 surround (mainly movies and some shows on HD channels).
LPCM (or PCM) is used for many stereo audio signals, but you may also receive stereo in the Dolby Digital format. Many people think that Dolby Digital is 5.1 surround sound - but DD can also be used for a simple stereo broadcast.
Therefore if you receive a stereo signal - LPCM or Dolby Digital - then your AV receiver will allow you to use virtual surround processing like Dolby Pro Logic or DTS Neo:6. Many people (well, me anyway) will have their receiver set to automatically switch to these modes as soon as a stereo signal is received.
However, if a true 5.1 surround sound Dolby Digital signal is received, then your AV receiver will automatically play this in surround sound around your speaker system - and you won't have the option of using Dolby Pro Logic or DTS Neo:6 (because you don't need them).
Dolby Digital stereo shouldn't sound much different to LPCM stereo - in fact if anything, the LPCM should sound slightly better as it isn't compressed like DD.
However, the Dolby Digital 5.1 surround should sound much better as the soundtrack has been mixed to work well on surround speaker systems. Therefore, the more true 5.1 surround sound audio you get from you TV broadcaster the better - but you don't have any choice in the matter.
This is why many people prefer watching a TV series on DVD or Blu-ray discs - as the sound will generally be better than that transmitted by the TV station.
As to your last point, it makes no sense that the signal changes if you reconnect the HDMI cable, as the audio format you are receiving from the TV station hasn't changed.
Hope that helps.
Paul (Site Editor)
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About Home Cinema Guide
Paul started the Home Cinema Guide to help less-experienced users get the most out of today's audio-visual technology. He has worked as a sound, lighting and audio-visual engineer for around 20 years. At home, he has spent more time than is probably healthy installing, configuring, testing, de-rigging, fixing, tweaking, re-installing again (and sometimes using) various pieces of hi-fi and home cinema equipment.