downmixing facilities

Hi. I am using mpc-hc in a win 7 laptop, hooked up to a 2-channel stereo amp and am using mpc to downmix dts hd ma and dolby truhd movie soundtracks into stereo.

The reported bitrate of the movie soundtracks is 5meg/second.

Question 1. when downmixed into stereo will the bitrate fall considerably, what do you think an average figure for that will be.

Question 2. Is it better to make mpc downmix or should i purchase an external processor perhaps in an effort to gain the best sound quality.

Many thanks in advance. John.

Downmixing with MPC? I hope you mean the audio decoder. Do not use the channel mapping options in MPC, that is wrong and not the same as downmixing.

This is correct:
Codec Tweak Tool > Audio Output
(also select Stereo in the Windows Control Panel)

After decoding the audio becomes uncompressed PCM audio. The bitrate is meaningless as an indication of quality. That only applied to compressed formats.

An external processor would be a waste of money. Downmixing is not a complex task.

The reason i ask is the dts hd ma soundtrack is very large 5,000 kb/s, and can only be sent by the hdmi connection, i have usb audio as well and digital coaxial but have been told these two do have sufficient bandwidth to send dts hd ma through.

Not sure if the downmixed stereo is 5,000 kb/s as well. It may be, or there may be some loss in the conversion process, just wondering if you could clarify in some way.

To set the sound to stereo i right click on the screen, select filters > lav audio > mixing tab > enable mixing > set output speaker configuration to stereo.

Under the status tab it states " Input codec dts hd ma. Output codec pcm channel 2/ 0x3 "

Thank you. John.

For stereo the bandwidth requirements are much lower because you only have two audio channels. S/PDIF can handle uncompressed stereo without problems. HDMI has more bandwidth.
HDMI versions

Hi Thanks for the reply, much appreciated.

Why do you say the bitrate in kb/s is of no consequence when determining sound quality, with a bitrate 0f 5000kb/s for blurays compared to 448 for dvd's surely there is more information coming through the speakers which would give more detail and clarity. If you feel bitrate is unimportant what is the spec to look for.

And how can spdif have the bandwidth to handle downmixed dts hd ma into stereo. If only hdmi has the bandwidth to handle dts hd ma, and hoping there is little loss in the actual downmixing itself the bitrate should stay at 5000kb/s.

Thank you. John.

Bitrate is only an indication of quality for a compressed format. A 320 kbps MP3 is better than a 128 kbps one. Once you decode the audio, you end up with uncompressed audio. The 'bitrate' of that uncompressed audio is unrelated to the bitrate of the original audio, so it would be the same for those two MP3 files. It is just a simple function of number of channels, sample rate, and bitdepth.

The audio can be send over the HDMI (or S/PDIF) connection in two ways. As uncompressed audio (PCM) or a compressed audio. The second case is called bitstreaming or pass-through. In that case both the hardware (audio chip and receiver) and the connection (HDMI) must support that specific audio format. It is not just a matter of bandwidth. When bitstreaming, the receiver is responsible for decoding.

You are currently decoding on your computer and thus sending uncompressed audio.

Sorry, not trying to be funny with you, i appreciate the reply i cannot understand the logic.

If the bitrate of a typical bluray is 5,000kb/s then when downmixed to stereo there should be little loss of information so what is the bitrate? it should still be pretty good.

I don't get why the size or amount of information, all the details and nuances of the audio, should decrease. If the audio track occupies 1-2gb which is typical of a bluray , and this is too big for spdif to handle, then why does it suddenly become ok to handle downmixed to stereo, the amount of information traveling through should be the same in both cases.

When decoding the audio, the quality stays exactly the same. No information is lost. The size increases because the data is no longer compressed.

When downmixing, you reduce the amount of audio channels. Information is merged from multiple channels into the two stereo channels. This results in a loss of quality. To be precise you lose the 'surround' effect of sound coming from different directions. But since you only have stereo speakers, this is not something you can avoid.

Example bitrate of uncompressed PM audio:
2 channels * 48000 Hz * 24 bits per sample = 2250kbps

8 channels (like the original DTS-HD MA) would equal 9000 kbps for PCM.

But the reason that SPDIF does not support DTS-HD is not bandwidth. It simply was not designed to handle that specific format.