AC3 encoder settings for reencoding.

[Mikel]

I know part of this is covered in the guides but i want to bring it up again to make sure everything is done correctly.

I looked through the forum about transcoding AC3 streams. With transcoding I specially want to make the bitstream smaller (reduce

it to 320 kbit/s or even 250 kbit/s).
I am aware that this might result in a more or less severe loss of audio quality.

I looked into some tools and made some tests with them.

I tried the following tools so far:
- BeSweet 1.4 with ac3enc.dll
- SonicFoundry SoftEncode 1.0
- Sonic Scenarist AC3 Encoder

The results as follows:

a) BeSweet 1.4 with ac3enc.dll and azid.dll
Good audio quality but with a pretty strong reduction in amplitude. For that reason it is not a good tool at the moment to

reencode. From the information i gathered in the forum, it must be some kind of basic problem with ac3enc.dll.
So ac3enc.dll is no alternative for me.

b) SoftEncode 1.0
SoftEncode seems to work ok, but it's terribly slow. It also sometimes crashes with some wave-files. I could not find the reason

for those crashes.

c) Sonic Scenarist AC3 Encoder
Seems to produce good results. However testing is still going on.

The question with b) and c) came up about what settings to use. a) is pretty straighforward with the settings.

So how do I configre a AC3 encoder working with streams (6 Audio files) that have been ripped from a AC3 stream in the first place.

In my understanding what could happen is that the "spacial impression" of the sound might be altered when reencoding with this

method. So I want to make sure that my settings are right so that I am as close to the 'original' AC3 stream as possible.

I will list the possible settings here and I hope someone can help me with the best settings. I indicated the settings with (ok)

that are clear to me.

Audio Production Service:
Data Rate settings: xxx kbit/s (ok)
Sample Rate: xxxxx kHz (ok)
Digital Normalization (??), possible settings are: -31dB to -1dB
Bit Stream mode (ok), i guess "main audio service" is usually right

Bitstream Information:
Center Mix Level (??), possible settings are: -3.0 dB, -4.5dB, -6dB
Surround Mix Level (??), possible settings are: 0dB , -3dB, -6dB
Copyright Bit: (??), yes or no, i guess this is not really important
Original Bit Stream: (??) yes or no, not really important either

Audio Production Information:
Audio Production Information Exists: (??), yes or no
Mix Level: (??), ranges from -80dB to -111dB
Room type: (??), small room, large room, not indicated

Preprocessing:
Input Filtering:
Digital Emphasis (??), yes or no
DC Highpass Filter (??), yes or no
Bandwidth Lowpass Filter (??), yes or no
LFE Lowpass Filter (??), yes or no

Surround Channel Processing:
Surround Channel 90° Phase Shift (??), yes or no
Surround 3dB Attenuation, (??) yes or no

Dynamic Range Compensation
Compression Characteristic (??), do i have to set none when reencoding?
RF Modulaton Protection: (??) yes or no

To make things clearer, I would like to include a screen shot but i am not allowed to post attachments. So i decribed it with words.

For me it's really important to have the right settings to _reencode_ a stream, _that has been encoded already_.

I would highly appreciate it, if someone could help me with the best settings for this task.

Cheers

Mikel