Is dithering always necessary?
hornblower
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#1
30th December 2011
Old 30th December 2011
  #1
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Thread Starter
Is dithering always necessary?

I have a little project now, digitizing old cassette tape copies of older (60s 70s) real-to-real tapes that have been lost. The quality varies, and some have a severe degree of hiss. I get rid of as much hiss as I can without destroying the point of having the recording, but - needless to say - some of the tapes are still pretty hissy.

Which leads me to wonder: is there any point in adding dither noise when I bounce from 24 to 16 bit? I understand that dither noise is 'uncorrelated' sound - would tape hiss do the trick? Then I wonder about line hiss and various background noise that I allow in other recordings I do. I did a recording for a guy the other day on a seriously windy day. He loved the subtle sound of the wind howling in the chimney and the humidifier-water pot simmering (hissing) on the wood stove and asked me to make it part of the sound. It gave a nice ambience and, again, I wondered if the dither noise was redundant.

Of course, with that much of a noise floor the dither noise is totally negligible so I used it anyway - but I can't help wondering if it is at all necessary in those sorts of circumstances.
jayfrigo
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#2
30th December 2011
Old 30th December 2011
  #2
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The short answer is that you should still dither from 24 to 16, and the dither noise will be so slight that it almost certainly won't be noticed under the tape hiss you describe. The alternative is allowing truncation distortion that is not only correlated thus more likely to be detrimental, but also has a greater peak value than the dither noise.

Self-dithering with noise present in a system has many limitations. First, it often doesn't have enough high frequency energy to completely decorrelate, and it also needs to be introduced during the recording or wordlength reduction. It can't simply be in the recording already. At that point, it's just part of your signal, your content. In other words, just because there is noise present in your recording doesn't mean it will act as dither. At that point it is no different from the vocal or the bass or the guitar; it's simply part of the recording.

Dither is not necessary is when you don't do any non-trivial processing. One example would be a gain change of 6.02 dB. This will simply be a bit shift, in other words everything moves one bit up or down and there is no complex math that expands the wordlength. It's rare to find software that will do this. a 6 dB change is not the same. Sonic Studio's soundBlade has a preference checkbox called "pure gain" whereby if you choose 6 or 12 dB etc., it automatically uses 6.02 or 12.04 to maintain a pure bit-shift rather than expanding the wordlength.


Quote:
Originally Posted by hornblower View Post
I have a little project now, digitizing old cassette tape copies of older (60s 70s) real-to-real tapes that have been lost. The quality varies, and some have a severe degree of hiss. I get rid of as much hiss as I can without destroying the point of having the recording, but - needless to say - some of the tapes are still pretty hissy.

Which leads me to wonder: is there any point in adding dither noise when I bounce from 24 to 16 bit? I understand that dither noise is 'uncorrelated' sound - would tape hiss do the trick? Then I wonder about line hiss and various background noise that I allow in other recordings I do. I did a recording for a guy the other day on a seriously windy day. He loved the subtle sound of the wind howling in the chimney and the humidifier-water pot simmering (hissing) on the wood stove and asked me to make it part of the sound. It gave a nice ambience and, again, I wondered if the dither noise was redundant.

Of course, with that much of a noise floor the dither noise is totally negligible so I used it anyway - but I can't help wondering if it is at all necessary in those sorts of circumstances.
hornblower
Thread Starter
#3
30th December 2011
Old 30th December 2011
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Thread Starter
Cool, that helps me understand dither better. I kind of figured as much, but it's good to hear it from someone who knows and understands.

Interesting about the pure bit-shift. So many crazy little details...

Thank you.
#4
4th January 2012
Old 4th January 2012
  #4
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Hi, maybe I'm late to this forum but was wondering if either of you knew the answer to this one.

How do I properly dither from say 96K or 88.2K/ 24 bit to 44.1/16bit in Pro Tools. I've heard that just "bouncing to disk", the conversion is less than optimal. Is there a great plug in one can get for pro tools that "properly dithers", i.e. accounts for discrepancies in dithering and matches them appropriately?
#5
4th January 2012
Old 4th January 2012
  #5
Quote:
Originally Posted by jayfrigo View Post
Dither is not necessary is when you don't do any non-trivial processing. One example would be a gain change of 6.02 dB. This will simply be a bit shift, in other words everything moves one bit up or down and there is no complex math that expands the wordlength. It's rare to find software that will do this. a 6 dB change is not the same....
Jay, just to clarify, if you are doing non-trivial processing, such as bouncing a track or a mix, or printing effects to a new track, but you are maintaining the 24-bit depth of the recorded material, is it advisable to dither?

My understanding is that the only time you want to dither is at the last stage, when creating the 16-bit cd stereo master file.
#6
4th January 2012
Old 4th January 2012
  #6
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Quote:
Jay, just to clarify, if you are doing non-trivial processing, such as bouncing a track or a mix, or printing effects to a new track, but you are maintaining the 24-bit depth of the recorded material, is it advisable to dither?
If the bit depth is not changing, there is no need to dither

Quote:
My understanding is that the only time you want to dither is at the last stage, when creating the 16-bit cd stereo master file.
Thats only true if your going from 24 bit to 16 or 32bit float to 16 bits. If the audio file is already at 16bit, then you do not dither since you are not going down in bit depth.
#7
4th January 2012
Old 4th January 2012
  #7
Gear interested
 

Only if you are going from 24 bit to 16 bit when pressing a CD. Dither is for keeping the quality and sonics of the song from degrading more when it processes down
#8
5th January 2012
Old 5th January 2012
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how does one properly dither?
#9
5th January 2012
Old 5th January 2012
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Quote:
Originally Posted by sluttygear View Post
how does one properly dither?
You dither at the end of the chain. It's the last part of the mastering if you do dither. Pow-R dither is amazing and so is Ozone's MBIT+ that's the best part of Ozone in my opinion
#10
5th January 2012
Old 5th January 2012
  #10
Banned
 

Quote:
Originally Posted by hornblower View Post
I have a little project now, digitizing old cassette tape copies of older (60s 70s) real-to-real tapes that have been lost. The quality varies, and some have a severe degree of hiss. I get rid of as much hiss as I can without destroying the point of having the recording, but - needless to say - some of the tapes are still pretty hissy.

Which leads me to wonder: is there any point in adding dither noise when I bounce from 24 to 16 bit? I understand that dither noise is 'uncorrelated' sound - would tape hiss do the trick? Then I wonder about line hiss and various background noise that I allow in other recordings I do. I did a recording for a guy the other day on a seriously windy day. He loved the subtle sound of the wind howling in the chimney and the humidifier-water pot simmering (hissing) on the wood stove and asked me to make it part of the sound. It gave a nice ambience and, again, I wondered if the dither noise was redundant.

Of course, with that much of a noise floor the dither noise is totally negligible so I used it anyway - but I can't help wondering if it is at all necessary in those sorts of circumstances.
I put 30 cassettes onto cd for a friend last fall.
played, digitised, inserted track breaks, adjusted levels so they were all at same max, and burned cd.
NO hiss noticeable.
was i lucky or were your levels off ?
or replaying way too loud?

but yes
tape hiss should be random enough to avoid problems
that dithering helps avoid. should not matter if you did it or not if it was full of white noise == unless you did other processing that could have caused correlation artifacts.
#11
5th January 2012
Old 5th January 2012
  #11
Gear addict
 

I always use a flat TPDF dither (type 2 in Ozone) or IDR type 1 with no shaping. Megabit + and pow-r are great too, but I am not a fan of noise shaping.
#12
7th January 2012
Old 7th January 2012
  #12
Gear maniac
 

Quote:
Originally Posted by sluttygear View Post
Hi, maybe I'm late to this forum but was wondering if either of you knew the answer to this one.

How do I properly dither from say 96K or 88.2K/ 24 bit to 44.1/16bit in Pro Tools. I've heard that just "bouncing to disk", the conversion is less than optimal. Is there a great plug in one can get for pro tools that "properly dithers", i.e. accounts for discrepancies in dithering and matches them appropriately?
You need to create a master fader track with the output you're bouncing from and insert dither plug-in there. Then Bounce to Disk as usual...
#13
9th January 2012
Old 9th January 2012
  #13
Quote:
Originally Posted by Rossd25 View Post
I always use a flat TPDF dither (type 2 in Ozone) or IDR type 1 with no shaping. Megabit + and pow-r are great too, but I am not a fan of noise shaping.
I'm not a fan of dither. I use noise shaping instead.
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