Welcome to Doom9's Forum, THE in-place to be for everyone interested in DVD conversion. Before you start posting please read the forum rules. By posting to this forum you agree to abide by the rules. |
18th February 2009, 09:54 | #1 | Link |
Registered User
Join Date: Mar 2008
Location: Australia
Posts: 246
|
How is Dolby Digital "true" 16 bit?
I was just curious how this claim stands true with a lossy encoder?
I understand with LPCM how this works as it is straight forward. The bits per sample, samples per second and the number of channels are a logical calculation when working out bitrate... 16 (bit) x 48,000 (kHz/samples per second) x 6 (channels) = 4,608 kbs. How is it that an AC3 (Dolby Digital) track can claim the same bit depth and sample rate, and only be a maximum of 640 kbs? I understand that this is a lossy encoder, therefore discards information to compress the data. However, if it isn't discarding bit depth or sample rate, what exactly is being discarded? Because 640 kbs simply isn't 16 bits per sample @ 48,000 samples per second by 6 channels! I am wondering the same thing about most of the lossy formats (DTS, MP3, AAC etc.) that state 16 bit, 24 bit, 48 kHz, 96 kHz etc. |
18th February 2009, 11:00 | #2 | Link |
User of free A/V tools
Join Date: Jul 2006
Location: SK
Posts: 826
|
Lossy means that sample values are being re-constructed on decoding from the internally binary stored (alias encoded) data. Information in these internal samples is somehow squeezed compared to their original value.
|
18th February 2009, 12:28 | #3 | Link | |
Moderator
Join Date: Feb 2005
Location: Spain
Posts: 6,890
|
Quote:
- Don't mistake samplerate with the bandwith codified. The encoders have limiter bandwith filters related to bitrate. - A sample in time domain have a bitdepth, but the samples are traslated to frequency domain when encoded, and the source bitdepth is lost. The samples in frequency domain are stored with a precission equivalent to 20-24 bits in time domain (AC3,DTS). The best option when decode an ac3/dts is use, at least, a bitdepth of 24 bits not matter what is the source precission.
__________________
BeHappy, AviSynth audio transcoder. |
|
18th February 2009, 12:42 | #4 | Link |
Registered User
Join Date: Dec 2002
Location: UK
Posts: 1,673
|
Though you won't get the "original" bits back, AC-3 can store a far greater dynamic range than can be represented in 16-bits.
It can happily store a sound peaking at 0dB FS one moment, and then one of -120dB FS the next. The latter sound would be lost in the dither noise (or rounded / truncated out of existence without dither) with 16-bits. Just because you don't get the original 24-bits back doesn't mean it can't make some use of 24-bits. Cheers, David. |
18th February 2009, 14:40 | #5 | Link |
Registered User
Join Date: Mar 2008
Location: Australia
Posts: 246
|
Thanks guys!
This all sounds very interesting. I certainly would like to learn more on a deeper scale which is why I posted this question here as I was hoping to open up a conversation on a technical level. I am certainly inspired to learn more about this. |
18th February 2009, 14:45 | #6 | Link | |
x264 developer
Join Date: Sep 2005
Posts: 8,666
|
Quote:
Works the same way in video. |
|
25th February 2009, 06:30 | #7 | Link |
Registered User
Join Date: Feb 2009
Posts: 8
|
Lossy audio (and video) codecs typically work in the frequeny domain, because its easier to take advantage of several features/limits of human perception.
Audio codecs typically use a modified discrete cosine transform. This tells them the amount of each frequency that's present in a series of very short, overlapping time slices. Within each timeslice, they can make decisions about which frequency detail is important to keep. Human hearing has some masking effects - we are not as sensitive to sounds (especially of similar frequency) which are temporally nearby or concurrent to louder sounds, for instance. Audio codecs take advantage of this to throw out information that they don't think will be noticed. This process can be done with any bit depth or time resolution. Increasing the bit depth increases the precision possible (it's possible to record finer changes). It is of course possible that this extra precision will be thrown out by the encoder due to bitrate pressure to encode more important data. I imagine bit rates higher than 16 bit are more useful for studio work where you shouldn't be using lossy compression anyway. Higher sampling rate, however is more important. Increasing the sampling rate does two things - it increases the frequency range expressible. The Nyquist limit states that the sampling rate must be at least twice the bandwidth of the signal, so if your signal has frequencies from 20 Hz to 20KHz, you need at least roughly a 40 KHz sampling rate to reproduce it. The other thing it does is gives the encoder more accurate frequeny information. The discrete cosine transform and other fourier-based transforms have the problem that the frequency information you get is uniformly distributed throughout the frequency spectrum, but that doesn't match how we see and hear. Since every octave change sounds like the same "distance" to us, but is actually a doubling in frequency, we are more sensitive to changes between low frequencies, so frequeny resolution is more important here. But in order to get better frequency resolution they would need to perform the transform on a larger block of data at a time, which leads to other problems (latency, higher memory and processing requirements on the decoder, harder to take advantage of some properties of hearing due to a corresponding decrease in temporal resolution). These are actually the problems that motivated the development and use of wavelets in codecs like JPEG2000 and Snow. |
25th February 2009, 12:31 | #9 | Link | |
Kid for Today
Join Date: Aug 2004
Posts: 3,477
|
last time I heard, AC3/DTS were able to output up to 18 bit if properly encoded(except for DTS 96/24 of course) :
http://209.85.229.132/search?q=cache...n&ct=clnk&cd=3 Quote:
Last edited by leeperry; 25th February 2009 at 23:26. |
|
26th February 2009, 04:25 | #10 | Link | |
Registered User
Join Date: Jul 2006
Posts: 276
|
Quote:
|
|
26th February 2009, 14:20 | #11 | Link | |
Registered User
Join Date: Mar 2008
Location: Australia
Posts: 246
|
Quote:
|
|
Thread Tools | Search this Thread |
Display Modes | |
|
|