I am a new forum member after having consulted this forum for many years on various audio/video issues.
This particular thread has been very informative, as I have recently "upgraded" to a flat-panel HD television and blu-ray player, but continue to use an older (i.e. ca. year 2000) surround sound amplifier with a single co-axial and single optical SPDIF input.
I can CONFIRM that - out of the box - my new (2012 model) LG 47LM5800 television will not pass-through any surround sound signal coming in through an HDMI port. All surround sound signals (Dolby Digital AC3 / DTS) are either mixed down or stripped to PCM linear (not sure which).HOWEVER, I can ALSO confirm that the service menu "hack" floating around on the internet for other LG televisions does allow the television to be set to passthrough a Dolby Digital surround sound signal from all HDMI inputs.
For instance, I have an LG blu-ray player hooked to my TV via an HDMI cord. The digital optical output of the TV is going to my surround sound amplifier. Prior to the service menu modification, the amplifier would only report it was receiving a linear PCM signal regardless of what type of audio track the blu-ray player was actually set to.
Once the service menu setting was changed to passthough Dolby Digital AC3, the Dolby Digital logo lights up on the amplifier's display whenever the blu-ray player is playing a disc with a 5.1 Dolby Digital audio track (i.e. a DVD or a blu-ray with non DD-HD track). Various speaker icons on the amplifier's display also light up depending on whether or not the Dolby Digital signal is 2.0, 2.1, 5.1 or whatever.
Use of the THX audio test found on a Lucasfilm Star Wars DVD confirms discreete DD5.1 audio is being passed through the TV to each speaker.
When I insert a regular audio CD into the blu-ray player, the DD logo disappears on my amplifier and it shows it's receiving linear PCM audio. Dolby Digital signals from my cable-company's PVR are also recognized by the amplifier according the multi-channel speaker information they contain.
The television will NOT pass-through DTS surround sound signals, however. When it receives a DTS singal the optical output is just mixed-down or stripped to become 2.0 linear PCM.
The LG TV also routes stereo analogue audio (from the component and composite inputs) through the digital output - which means audio from my Nintendo Wii console hooked directly to the TV gets routed to my amplifier (nice!). It also seems to preserve the Dolby Pro-Logic II surround signal being output from my Wii console - as I can hear seemingly discreete audio effects in the rear speakers. This would need more testing to confirm.
Editing the service menu was fairly easy. This link was helpful:
This PDF document - although not for my particular model, was similar enough to also be helpful:
You go into the service menu option "AC3 EDID D/L" and choose "START", then all HDMI and RGB inputs will change their status from "OK/(PCM)" to "OK/(AC3)". Exit the service menu, shut down and and turn the TV on again. That's it.
I was able to use a cheap RCA universal remote to enter service mode as described in the first link above.NOW, I have one question:
when the LG TV recieves a non Dolby Digital signal (i.e DTS or other multi channel digital), it passes it through as linear PCM. My question would be - is ANY surround information preserved in this linear PCM mixdown, or is it all stripped away to a stereo left/right 2.0 signal
? I am HOPEFUL that the TV is at least smart enough to create a PCM stereo signal containing some surround information matrixed into a Dolby Pro-Logic signal. However, I realise it's probably - disappointingly - just stripping all the surround info out (or mixing it down to a simple Stereo left/right signal.
When my amplifier recieves one of these "mixed down" linear PCM signals from my TV, I do hear some activity from its back speakers. But I'm not sure if what's being sent from the TV is a true Dolby Pro-Logic signal or if, instead, the Dolby Pro-Logic II circuit in my amp is just trying to create a simulated surround sound signal from a basic two channel stereo signal (as this is what I've read Pro-Logic will attempt to do when it doesn't receive a true Pro-Logic matrixed signal).