1. What does PCM mean and how does the sound quality differ to digital? My old ears can't appreciate the improvement.
2. Could a cheap HDMI cable connecting the TV(ARC) to the soundbar cause a sync delay?
PCM is the digital version of a stereo signal. The digital connection (optical / RCA coaxial / HDMI) delivers two channels of audio. For TV audio, we can think of PCM as similar in quality to CD audio. It is a full range audio signal (20Hz~20kHz) and doesn't use any form of compression, so there's no loss in quality.
PCM stereo can have surround information added to it by using
Dolby Surround encoding. That adds a centre channel and rear surround channel in a way that keeps it hidden when the signal is played through a basic stereo-audio system, but it can be extracted by using a Dolby ProLogic decoder (DPL). This tech has been around in consumer equipment since the mid-80s. It had a revamp in the early 2000s to become Dolby ProLogic II decoding (DPL II). This improved the separation of the decoded channels to make the centre dialogue more distinct. It also created a steering effect for the rear surround
What you refer to as
digital also goes by the names B
itstream.,
Dolby Digital and
Raw. In most cases it is five channels of full range audio (20Hz~20kHz) plus a limited frequency deep bass channel (3Hz~200Hz) called LFE. This stands for Low Frequency Effects. Together this comprises a 5.1 audio signal. You might see it referred to as
DD5.1 or just
DD. The whole lot is then encoded into a single data stream, hence
Bitstream.
Because each audio channel is discrete from its neighbours, the separation of the channels is clearer than with the matrixed form decoded by DPL/DPL II. The positioning and steering of effects has greater precision.
DD doesn't have to be 5.1. It can be mono (1.0), stereo (2.0), stereo +LFE (2.1), or any combination up to 6.1 where 5.1 gains an extra channel to the rear to make 6 surround channels + LFE. All of these systems combinations though use lossy compression. What you gain in steering precision has a cost in terms of fidelity. It still sounds decent, but DD 2.0 is no match for PCM stereo.
Your second question was whether a HDMI lead could cause this lip sync error. The answer in most cases is no unless it's a very old Standard Speed HDMI cable.
In most cases the lip sync error is cause by the time taken by the TV to process picture and sound.
These sync issues have become more noticeable as TV resolutions have increased from 1080p to 4K (more image processing time required for the upscaling of SD and 1080p video. There's also a delay caused if a lot of image processing such as image sharpening, contrast enhancement, motion smoothing, black frame insertion etc is going on.
On the audio side, any TV set to a mode where it decodes audio then has to re-encode for the output to a sound bar or AV amp is going to introduce some delay. Set the TV to Pass-thru if the sound bar or audio system is something with more than two channels. Setting the audio to PCM means that the TV is forced to switch off a lot of audio conversion processing. Where the TV is say dealing with a multichannel broadcast but then trying to convert it to stereo for its own speakers as well as trying to accommodate an external multichannel audio system is asking too much of it.
It's also possible to encounter errors if the TV has gaming modes (ALLM or VRR) enabled or left on auto for inputs that won't use them.
