Do Macs have auto gain on DVD playback software?

PeterWright wrote on 2/26/2009, 6:51 PM
A friend has produced a DVD of a theatrical performance, shot on HDV, and the client has complained that when he plays it back on his Mac, there is a noticeable rise in ambient noise during quiet periods. My friend has listened carefully on PC and set-top playback, and cannot detect this at all.
I've not come across this before - has anyone else?

Comments

newhope wrote on 2/27/2009, 3:33 AM
First thing to know is that, unless the Mac is connected via a digital (optical) cable to an amplifier with Dolby Digital capability it is going to convert any AC3 (Dolby Digital) track to stereo through down conversion in the Dolby Decoder software which is part of the DVD Player Application.

The default for the player is to have 'Dolby dynamic range compression' ON.

The end user can open the 'Preferences' for the DVD Player and tick a box to 'Disable Dolby dynamic range compression'

Whether this should be necessary gets back to how your friend encoded the audio for the DVD.

If they have output an AC3 track as a separate render from Vegas, then setting the dynamic range compression setting to 'none' in the Dolby Digital Pro Encoder should avoid any subsequent Dolby dynamic range compression being applied.

In Vegas this setting is defaulted to ON in the Dolby Digital (AC3) Pro encoder and is permanently ON the standard AC3 encoder (can't remember what it is called but it's the alternate choice in Vegas and I'm currently posting from my Mac to answer the DVD Player question.)

It's normally set to 'Film Standard' compression which only gets activated when a downmix is applied to the original AC3 track during replay. I normally set the option to 'None' to avoid any dynamic range compression being applied automatically during replay where downmixing is applied.

So the simple answer is YES there is dynamic range compression in the Mac DVD Player which by default is ON... BUT it can be turned off.

In fact there is dynamic range compression in every Dolby Digital decoder in every player, be they software or hardware. Whether it is defaulted to ON automatically is dependent on the manufacturer's default setting and the end user's choice/options to override this setting.

Whether it will be applied in replay gets down to whether the replay system will require down mixing of the original AC3, i.e. 5.1 down mixed to stereo. Even AC3 stereo to stereo line output replay can activate the DRC settings if they are set in the bit strream settings during the AC3 encode.

So tell your friend to make sure they disable dynamic range compression when outputting the AC3 file from Vegas and ALWAYS use the AC3 Pro Encoder that allows this to be done.

Regards

New Hope Media