The flaws of blind listening tests

[John Dyson]

1 hour ago, gmgraves said:

I used to have  a Dolby-A unit that I used for all my analog recordings. I recorded a pro symphony orchestra for 5 seasons using it as well as various jazz venues (including the San Francisco Jazz Festival) for the NPR "Jazz Alive" series. Aaron Copland, Hubert Laws, Dizzy Gillespie, and Stepan Grappelli are among the luminaries that I recorded. I used 15 ips (38cm/sec) and half Track stereo using Dolby-A for all of these. I'm pretty sensitive to distortion and what I found is that if the operator hasn't been scrupulous with matching the levels with Dolby-A on both the record and playback cycles, it will introduce distortion. My opinion is that this is fairly straightforward to do, but it is time consuming and I can see many technicians getting sloppy when they have to adjust separate Dolby units for as many as 48 tracks!. I'm going to venture a guess and say that this is probably where the notion of Dolby-A (and possibly DBX with which I have no experience) introducing distortion came from. I was only recording in two channel, and therefore took the time to make sure that my R-to-R tape decks were properly and meticulously set up and paired to my Dolby-A unit. 

Gotta say, when digital recording came along, I was happy to put those laborious chores behind me!

You are definitely right about DolbyA creating distortion when not calibrated correctly and I cringe if someone has multiple channels improperly calibrated trying to line everything up.

However, there is a deeper form of distortion that is created by any fast compressor or expander.  The DolbyA has the potential of a 1msec attack time at frequencies about 3kHz-20kHz (actually, the 3kHz band includes frequencies down to about 1kHz plus or minus.)  The 20-80 and 80-3k bands can have as fast as 2msec attack time.  Those fast attack times in cases of intense material will produce rather large amounts of transient distortion possbily all the way up to 30-40kHz.  In fact, if you look at the spectgram before and after a DolbyA encoding operation, you might notice a significant increase of above 20kHz material.  Much of that increase of 'splats' as I call them result from the mixing of the 1msec attack (with lots of harmonics) with the multiplication (gain control) of the audio signal.  This creates mixing products that are distortion.(The mixing is similar to the action of an RF mixer as used in radio recievers.)

So, when you get that old recording that had been DolbyA encoded/decoded (or from many other HW noise reduction systems -- not just DolbyA), and you see all of that 'nice' above 20kHz energy....  Not all of that energy comes directly from the music, but at least some (if not most) comes from the noise reduction system.

The kind of distortion that is created by DolbyA appears to be a softening of the audio (that is the sense of the distortion.)  The 'splats' don't help with the compatibility with equipment that doesn't deal well with excess HF energy.

 

John

[gmgraves]

While I do not doubt you in the least, I must say that I have never noticed this distortion on any of the recordings I made using Dolby-A. However the kind of distortion that you speak of; the "splats" as you call them, are very noticeable with FM broadcast compressors The one-time ubiquitous CBS Automax compressor (and the Fairchild compressors before it) had this sound in spades. Bob Orban of Orban Associates worked extremely hard when developing his original "OptiMod" compressor/limiter/multiplex stereo generator. He found (IIRC) that a fast attack and slow release cut down on this type of distortion (which in FM, is , of course, a single-ended compressor, and not a compander like the Dolby or DBX noise reduction systems). So, I am aware of the kind of distortion that you are talking about

[John Dyson]

2 hours ago, gmgraves said:

While I do not doubt you in the least, I must say that I have never noticed this distortion on any of the recordings I made using Dolby-A. However the kind of distortion that you speak of; the "splats" as you call them, are very noticeable with FM broadcast compressors The one-time ubiquitous CBS Automax compressor (and the Fairchild compressors before it) had this sound in spades. Bob Orban of Orban Associates worked extremely hard when developing his original "OptiMod" compressor/limiter/multiplex stereo generator. He found (IIRC) that a fast attack and slow release cut down on this type of distortion (which in FM, is , of course, a single-ended compressor, and not a compander like the Dolby or DBX noise reduction systems). So, I am aware of the kind of distortion that you are talking about

The DolbyA distortion doesn't really sound 'bad', but tends to soften the sound -- and that is one reason why it hasn't been very aggressively fixed.  DolbyA encoded/decode CAN cause a lost 'precision' in the sound (gawd -- hate to use terms like that :-)).

Wrt simple anti-distortion mechanisms...  R Dolby acutally used a rather ingenius and simple one on the DolbyA (but it still becomes too fast), and that is that crazy diode network that also doubles as a rectifier.  (If you look at the 361/cat22 schematic and see the 4 JFET compressors -- the fancy diode network is on the right side of the compressor circuit.)   That circuit should be looked at SUPER carefully, because he actually made the attack slow for small increases in signal level, then speed up rapidly for fast increases.  It was a cool design with his very careful way that he finessed the log/exponential characteristics of the diodes, when the know-how was a little less common in 1965.

 

* One manifestation of DolbyA distortion is when there might be a chorus of two, three or four people, and somehow their distinct voices are softened into a chorus (it really isn't 3-4 clearly distinguishable voices anymore.)  That is one commonly manifested kind of distortion occuring on DolbyA.  The effect is similar to tape compression (which is a form of distortion), where it softens the sound in a mysterious way.  A good group to check for the distortion might be Simon & Garfunkel when they are singing clearly without much accompanyment.

 

WRT Orban -- I think that he is the one with the phasing patent.  I use a similar technology (but super different design) in my decoder to cancel out practically all of the distortion (it really doesn't get every last bit, but it does amazing things.)   I always avoid infringing on patents, however I really didn't consider his approach at all for what I have been doing.  (His thing is really cool also.)  Using his phasing concept can be helpful in hiding some of the IMD from fast limiters (and it actually implements part of the fast limiter.)  Doing what Orban (I do hope he is the person with that patent) did back in the 1970's was pretty good forward thinking also.

 

I am working very hard right now, trying to get a semi-final release ready.  Needed a break, so did some forum trolling (hopefully not in a negative way!!!!)

 

John

[fas42]

Some recordings are badly distorted, or very noisy. For a variety of reasons; a degraded master tape or a well worn 78 as the only archive we have available are a couple of obvious examples. So, how are we "to deal with that"? One method is to attempt to salvage the material, reverse the damage by some process - Cedar is a well known company doing that, and John's efforts here - to create a "new mastering".

 

Which may work for some folks. An alternative is to improve the capability of the playback rig, allowing one to "hear through the distortions and noise"'; IME this works extemely well, and means that none of the surviving capture of the musical event has to be discarded, or mutilated, to achieve a positive listening experience - it's a win-win.

[gmgraves]

21 hours ago, John Dyson said:

The DolbyA distortion doesn't really sound 'bad', but tends to soften the sound -- and that is one reason why it hasn't been very aggressively fixed.  DolbyA encoded/decode CAN cause a lost 'precision' in the sound (gawd -- hate to use terms like that :-)).

Wrt simple anti-distortion mechanisms...  R Dolby acutally used a rather ingenius and simple one on the DolbyA (but it still becomes too fast), and that is that crazy diode network that also doubles as a rectifier.  (If you look at the 361/cat22 schematic and see the 4 JFET compressors -- the fancy diode network is on the right side of the compressor circuit.)   That circuit should be looked at SUPER carefully, because he actually made the attack slow for small increases in signal level, then speed up rapidly for fast increases.  It was a cool design with his very careful way that he finessed the log/exponential characteristics of the diodes, when the know-how was a little less common in 1965.

 

* One manifestation of DolbyA distortion is when there might be a chorus of two, three or four people, and somehow their distinct voices are softened into a chorus (it really isn't 3-4 clearly distinguishable voices anymore.)  That is one commonly manifested kind of distortion occuring on DolbyA.  The effect is similar to tape compression (which is a form of distortion), where it softens the sound in a mysterious way.  A good group to check for the distortion might be Simon & Garfunkel when they are singing clearly without much accompanyment.

 

WRT Orban -- I think that he is the one with the phasing patent.  I use a similar technology (but super different design) in my decoder to cancel out practically all of the distortion (it really doesn't get every last bit, but it does amazing things.)   I always avoid infringing on patents, however I really didn't consider his approach at all for what I have been doing.  (His thing is really cool also.)  Using his phasing concept can be helpful in hiding some of the IMD from fast limiters (and it actually implements part of the fast limiter.)  Doing what Orban (I do hope he is the person with that patent) did back in the 1970's was pretty good forward thinking also.

 

I am working very hard right now, trying to get a semi-final release ready.  Needed a break, so did some forum trolling (hopefully not in a negative way!!!!)

 

John

Bob Orban and I were casual friends at one time, but I haven't talked to him in more than 30 years. I used to visit a friend who was a programmer at a local FM station. He played classical music from about 7 in the evening until about 6 AM every day. I'd stop by several times a week and both of us being audiophiles, we'd BS for hours while long symphonies played over the air. Bob maintained his "laboratory" in that radio station and worked on the "Optimod" there. This station, KPEN in Mountain View, CA,  was the one he used to test his designs. Unfortunately, I know little about his design, as he understandably played his work pretty close to the chest.