4K HDR HTPC - intermittent bitstream audio dropping out

Hi folks,

I have recently built a new HTPC for the purpose of 4K HDR playback.

My system configuration is brand new, and is as follows:

Display: LG OLED 65C7T
CPU: Intel coffee lake i7-8700 (containing Intel Graphics HD 630)
Motherboard: Gigabyte Z370 Wifi
RAM: 16GB RAM
AVR: Yamaha RX-A3070
HDMI 2.0a connector from the motherboard to AVR then to TV. High-Speed HDMI cable - it is a few years old, but states "high speed" on the cable itself.
Content: 4K HDR MKV files - BT2020 colour range; Audio is Dolby Atmos/TrueHD track (but also play other content at 1080p and less)
Media Player: MPC-HC (1.7.16.2) using MadVR (0.92.14) as the Video renderer and LAV Video/Audio/Splitter (0.71.0.34-git) - all audio is passed through as bitstream to my Yamaha - all sound formats.
Hardware Decoding chosen: Direct3D11. CPU "load" is less than 5%, GPU ranges between 60-80%

My old HTPC (1080p capable only - i-2700K based) I would have my TV set at 50Hz and all my movies are generally at 23.976 fps but everything would play perfectly, no audio issues, no visible frame loss issues, etc.

With this new much more powerful HTPC I again, set the TV at 50Hz and noticed audio dropouts - every few minutes for about a fraction of a second. The on screen display of my Yamaha would blink when the sound was lost, and then regain its display on whatever format it was - i.e. DD for Dolby Digital, ATMOS/THD for TrueHD/Atmos.

I then played with the TV refresh rate, setting it to 23Hz for 23.976 frames per second, and I have landed on the following use cases, and wouldn't mind some insight.

Here are the two situations I’m facing involving playing 4K (2160p) HDR (BT.2020) content (but it has the same scenario if it was a standard 1080p MKV and sound mix is not a factor either - i.e. DTS, Dolby Digital, Atmos, DTS-X - it doesn't matter):

Situation 1: 23 Hz refresh rate selected (with Intel Graphics HD 630), content to be played is standard 23.976 fps:
Audio: Perfect, uninterrupted audio via bitstream – Dolby Digital/TrueHD/Atmos or DTS-HD MA or DTS-X – no issues at any time.
Video: Nearly perfect. Occasionally get some dropped frames, can be noticeable but happens rarely, yet it still bothers me - I never used to have this problem.

Situation 2: Set a 50 Hz or 60 Hz refresh rate on the TV, content to be played is standard 23.976 fps:
Audio: drops out momentarily every few minutes – no specific time interval or pattern. It doesn’t relate to action on screen, nor does it relate to sound mix – can be DTS, Dolby, and any variant thereof.
Video: Absolutely perfect – zero dropped frames apart from when you start the video there’s naturally a moment where the picture and audio synchronise, then after that – it’s perfect.

What I cannot understand is why it appears that refresh rate is affecting audio. This is something I’ve never experienced before on my old HTPC that did only 1080p.

Can anyone shed some light on why this is happening? Having my refresh rate at 23hz makes moving the mouse and browsing painful, and I have never had an issue with 50Hz refresh rate and 23.976fps content playing on a much older less powerful PC, admittedly, I am asking this PC to do a lot more with 4K, but it does not matter what the file is - small 1080p 1gb file or large 4k 10gb file - the dropouts are the same.

I feel that it is the picture reproduction and processing that somehow is affecting audio reproduction but I cannot understand why and how to address it.

Best wishes and thanks in advance.
Mike.

These symptoms occur when you reach the bandwidth limits of the HDMI connection. HDMI 2.0a is unable to transmit high bitdepth 4k video at 60Hz. I don't remember the exact numbers right now, but even 50Hz is near/above the limit.

One solution is to use 23Hz. Madvr has the ability to automatically switch between refreshrates, based on the content you play.
Madvr settings > Devices > display modes

It also has the ability to create optimized custom refresh rates. That way the amount of frame drops/repeats gets reduced significantly.
Madvr settings > Devices > display modes > custom modes
Make sure to have a different refreshrate active (60Hz) when creating an optimized one. The standard optimized ones are pretty good already. If that is not enough, then you can click 'optimize'. Then it will calculate the optimal values while watching a video.

Another solution is to configure the driver to output 8 bit video instead of 10/12 bit.

Admin wrote:These symptoms occur when you reach the bandwidth limits of the HDMI connection. HDMI 2.0a is unable to transmit high bitdepth 4k video at 60Hz. I don't remember the exact numbers right now, but even 50Hz is near/above the limit.

One solution is to use 23Hz. Madvr has the ability to automatically switch between refreshrates, based on the content you play.
Madvr settings > Devices > display modes

It also has the ability to create optimized custom refresh rates. That way the amount of frame drops/repeats gets reduced significantly.
Madvr settings > Devices > display modes > custom modes
Make sure to have a different refreshrate active (60Hz) when creating an optimized one. The standard optimized ones are pretty good already. If that is not enough, then you can click 'optimize'. Then it will calculate the optimal values while watching a video.

Another solution is to configure the driver to output 8 bit video instead of 10/12 bit.

Thanks so much for the quick reply!

First item to cover is related to the auto refresh rate suggestion. I have tried to implement this, however the issue I'm facing with this is that when MPC-HC starts up and the TV goes black whilst it switches refresh rate, for some reason the HDMI connectivity interrupts on my receiver (whilst this transferral takes place) - and seems to "point" back to the TV. I therefore no longer hear any sound from my receiver as it is not decoding bitstream any more. The way to "snap" it out of this is to go to the volume and modify the volume to force a sound out of the PC which points the receiver it back to the HTPC again but I have to exit MPC-HC to do this. I have tried turning off all ARC capabilities, HDMI CEC control (on my Yamaha receiver and my LG TV) - but because of this refresh shift, the sound doesn't work, so I can't do this auto shift, unfortunately. A solution to this problem would be welcomed!

The second point to cover is related to the bandwidth question. I will investigate and ensure my cables are suitable (they are no more than 4 years old, I'd say) - but I don't get any bursting or other of the symptoms of a poorly functioning cable. The only concern regarding this aspect is that this scenario happens regardless of the file. It happens on an old 720p file, 480i files, as well as 1080p and 4K. I realise that this is being upscaled, so ultimately the load might be the same regardless of the file size/bit rate, so I'm unsure whether this aligns with the bandwidth scenario. Similarly, I'm not talking about movie files that are shot in 50fps. All my content is 23.976 and in some cases for local productions, 25fps (I'm in Australia) - so does the bandwidth requirement increase because my refresh rate is at 50Hz even if the fps is still 23.976?

Finally, I will investigate further how to use the optimisation scenario you describe above - if there are any good guides telling me how to do this (or other optimisation settings) that would be tremendous.

Thanks again for your quick reply!
Cheers,
Mike.

I am not sure how to solve the first issue. But there are some things to try. In Windows sound settings > Device > Properties > Advanced, there is an exclusive mode settings. Try turning that off (or on).
Or manually change resolution before starting playback.
http://tools.taubenkorb.at/change-screen-resolution/

After checking, HDMI 2.0a should have enough bandwidth (plain 2.0 does not). But of course every element in the chain, including TV must have that connection.