Reencoding AC3 - Phase Shift

[RRAH]

Hi there,

I have a question about the following workflow:

Decode AC3 to WAV - some Changes - Encode WAV to AC3

Should I enable "90 degree phase shift" when encoding the new AC3 or have I then a "180 degree phase shift"?

What about DTS to WAV - some Changes - Encode WAV to AC3?

Thanks a lot

[tebasuna51]

If surround channels are already phase shifted, in AC3 or DTS, don't need other shift.

[Ghitulescu]

I believe if one uses phase shift he should refer to anything else than 180° (or 90°). When a sound engineer hears of phase shift he would think also on temporal displacement of a particular spectrum (say trebles) vs another particular spectrum (eg bass) - because this plagues analogue gear. He would never think of reversed phase (or antiphase).

180° is the reverse phase in other words when the sound of one channel is in antiphase to the other ones (for multiple channels read combinations of). This is mainly due to wrong cabling (for analogue sounds) or wrong starting point for digital synthesised sounds.

Anyway, a phase difference of max. 90° (between bass and trebles or between channels) is allowed, provided it serves a purpose (eg the phase difference between channels emphasis the stereo/spatial effect). In other words 90° shifting renders (or cancels :) ) a stereo effect, a phase shift of 180° makes almost no difference (see below).

I am not aware of audio tools within purchasing power of an individual that would change the phase in the musician's understanding - because such a tool would remove cinavia as well :) .

BTW, phase inversion is almost inaudible. How did you find you have one (and therefore need to alter it)?

[raffriff42]

Dolby Digital Professional Encoding Guidelines
http://www.beussery.com/pdf/beussery.dolby.pdf
(c) 2000 Dolby Laboratories Inc 4.7.5 Surround Channel 90-Degree Phase-Shift

The Surround Channel 90-Degree Phase-Shift feature is useful for generating multichannel Dolby Digital bitstreams that can be downmixed in an external two-channel decoder to create a true Dolby Surround compatible output. This parameter is user-adjustable only when Surround channels are present in the bitstream, i.e., only when Audio Coding Mode is set to 2/1, 2/2, 3/1, or 3/2.

The 90-Degree Phase-Shift parameter should always be left enabled except under specific conditions. These include, but are not necessarily limited to, system calibration, encoding of certain test signals, and in the extremely rare case when the discrete playback of highly coherent program material may be compromised.

D.1 90-Degree Phase-Shift Filter

The 90-Degree Phase Shift filter provides a means for an encoding engineer to create a multichannel Dolby Digital bitstream that can be downmixed to a Dolby Surround-compatible Lt/Rt output. Without this filter, point-source elements panned from Surround to Center in the multichannel mix would seem to pan from Surround to Left and then to Center when downmixed to Lt/Rt and reproduced using a Dolby Surround Pro Logic decoder. This filter should generally be used whenever encoding a multichannel signal unless it is known that the 5.1-channel source does not contain point-source element pans. For example, if the source was recorded using five discrete microphones placed in the corners of an auditorium, there is no panning between channels and the filter could be safely disabled. If in doubt, use a DP562 to downmix the 5.1-channel program to Lt/Rt, Dolby Surround Pro Logic decode the Lt/Rt signals, and then set the filter to the setting that sounds best.

Description
The 90-degree phase-shift is created using a very long FIR filter. Since this filter introduces a significant time delay, the other four channels are delayed using a PCM delay line so that all six channels are kept in sample alignment. This filter has exactly 90-degree phase shift at all frequencies. The magnitude response is flat across most of the spectrum, rolling off at the lower edge of the audio band (-3 dB below 30 Hz).


YOU ARE SURROUNDED, Part 5
http://www.soundonsound.com/sos/dec01/articles/surround5.asp Metadata & Downmixing

By default, the Dolby Digital encoder introduces a 90-degree phase-shift between the two surround channels, so that when they are downmixed and the left surround is added to the left front, and the right surround to the right front, they automatically produce a Pro Logic-compatible matrixed surround channel. This phase-shift, although not required in a discrete 5.1 system, is usually inaudible with most material. However, it can become apparent with some music-only recordings, so the phase-shift can be disabled during the encoding if necessary. This is then flagged in the metadata so that it is then impossible for the decoder to produce an Lt/Rt downmix. Clearly, it is therefore important to check the Lt/Rt and Lo/Ro downmixes of the material at the mixing stage, and flag the best compromise in the metadata.Edit: If surround channels are already phase shifted, in AC3 or DTS, don't need other shift.Agree. But try it and see: listen to the re-encoded and decoded output in all modes - discrete surround, Dolby surround, stereo and mono. You may have to compromise - meaning decide which mode(s) you care about.

[pandy]

IMHO this is metadata problem at this stage as downmix type (Lo/Ro or Lt/Rt was selected at source encoding) so to not change anything it is better to just copy metadata.

Selecting particular downmix mode is done anyway in reference to the decoder mode (RF/Theater etc) - there is lot implications to additional processing for such prepared audio.