This is topic "warbling" Dolby Digital? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Jesse Skeen (Member # 586) on 02-05-2001, 01:37 AM:
I thought I was going nuts when I heard it do this, but someone else noticed it too- on one screen the Dolby Digital has a 'warbly' sound to it, like that of a tape sticking- it sounds like it is very quickly slowing down then coming back to regular speed. Has anyone else had this happen? We're using the penthouse reader and I'm guessing that's what the problem is; the same screen also has SDDS which doesn't do this and the analog sounds fine too. I didn't think it was even possible for a digital format to do this though.
Posted by Josh Jones (Member # 447) on 02-05-2001, 09:28 AM:
The sound you hear is a result of the compression Dolby uses. If you listen to an MP3 you will notice this also. The type I'm used to exists in the treble part of the spectrum. It is caused by the algorythm dolby used for perceptual coding. it is normal. That is why I cant listen to MP3'sJosh
Posted by Darryl Spicer (Member # 711) on 02-05-2001, 09:38 AM:
Does this happen on all the prints that you play in this auditorium. The print that yau are playing currently. Does the problem move with it when you move it. Do you have more than one dolby digital system. If it moves or does not present the problem with other films it may be an actual badly compressed digital sound track or defective print reel if it does not do it all the way thru the movie.
Posted by Scott Norwood (Member # 30) on 02-05-2001, 09:55 AM:
Yes, I'd be interested to know if this happens only with one print or with every film in that auditorium.
I'm betting that it only happens with one print and that it's a bad compression job.
To my ears, the Dolby compression scheme is much better than MP3. More importantly, perhaps, it's better than the alternative (Dolby SR optical), whereas MP3-compressed music sounds substantially inferior to the alternative (a CD or good-quality LP). And, yes, I hate MP3 as well...I'm really quite surprised that the record companies see this as a threat. But I digress...
Posted by John Walsh (Member # 168) on 02-05-2001, 11:17 AM:
The AC3 compression algorithm Dolby uses is not *that* bad. I feel there is just something wrong with that print, or maybe with the unit itself.I thought there were different compression settings for MP3. When I played with it, the highest compression sounded crappy, with a sort of echo to it. The lowest sounded just OK. MP3 does seem perfect when you're just crusing around in the car, or for background "fill."
I was impressed with Dolby's honesty about AC3. At a tech seminar I went to, the instructor made it quite clear that Dolby was not proclaiming AC3 as the best thing since sliced bread. Their position was that they wanted one scheme that several manufacturers would go for, would work for several formats, (DVD, 35mm film, HDTV, etc.) and provide 5.1 channels and reduce to 360kbs. I think all thing consitered, they did a good job.
Posted by Josh Jones (Member # 447) on 02-05-2001, 01:39 PM:
Ok, perhaps I was a bit harsh about Dolby's Algorythm. I only notice it When I am very close to the stage speakers. On the subject of MP3's I have heard a few that sound as good as the origional. most of the distortion and Mpeg splatter, as I like to call it, is caused by the ripper itself. It all depends on how and what it compresses or eliminates from the origional samples.Josh
Posted by Evans A Criswell (Member # 381) on 02-05-2001, 02:03 PM:
I have never experienced a "warbling" sound from a Dolby Digital soundtrack, either in the theatres or at home. The lossy compression algorithm issue is a very interesting one. I have made many MP3 backups of Top 40 hits in my collection so if something happened to my CD collection, I would still have the songs.I've always wanted to do an A/B comparison of MP3 with original CDs, but there is no easy way to do it on my system because when I play CDs, my CD player simply sends the bitstream into my receiver (Yamaha DSP-A1) for decoding. With MP3, my computer's soundcard does not have a digital output, so I'd be sending analog in, which would get redigitized, which is not a fair comparison. I suppose I could convert some MP3s back to WAVs and put them on CD and play them on my system for the comparison. I use 256kbit/s for better quality.
Dolby digital is 360 bits/second in theatres, right? That encodes 5.1 channels of information, so I would think that the sound quality of a 5.1 Dolby Digital soundtrack would be equal to the sound quality of a 70 kbit/second mono track (140 kbit/second for stereo). It seems that the MP3 rate needed to beat DD in fidelity for stereo music would have to be above 140 kbit/second, assuming the algorithms are roughly the same in quality. DD at home is usually 448 kbits/second, so it seems you'd need 175 kbits/second to beat the sound of that for regular stereo music.
Evans
Posted by Antonio Marcheselli (Member # 373) on 02-05-2001, 05:44 PM:
Josh,
I heard Dolby Digital "warbling" very clear on "Fantasia 2000" trailer: with classical music is simpler to hear the Ac3 compression.
However I'm sure that that trailer was made very bad... I can hear that DTS is more cristal clear than DD but I cannot say that I'm able to hear the DD compression on good DD prints... 
Bye
Antonio
Posted by John Walsh (Member # 168) on 02-05-2001, 09:31 PM:
For the techies... From the Dolby AC3 Encoding Manual:12:1 compression (typical)
All main channels- full range
LFE channel- 3hz to 120hz
It looks like the actual bit rate changes, depending on the use. For example, the bit rate when used in theaters is 320kbs, but the rate is 448kbs with a DVD.
Also, I don't think many people realise that AC3 provides several extra features not used in a theater. For example, from a DVD at home, the AC3 provides a "smart-mixdown" depending on the format. When you switch the DVD player audio settings from, "5.1 channel" to "mono" the actual mix can be changed. If the director feels that a person listening in mono should not hear what was in the surround channel, he can remove it. If a person with poor hearing is listening, they can switch to a pre-approved mix that has dialog-only. There's bits in there to tell the listener what kind of room the sound was mixed for, (large dubbing stage or smaller recording studio) that the surround channel is or is not being used, a karaoke mode, copyright protection, etc.
We don't care about some of those features when at a theater, but it does make it a universal format for several applications.
Posted by Brad Miller (Member # 2) on 02-05-2001, 09:36 PM:
I have seen this happen in one DA20 about 2 years back. It was speeding up and slowing down "out-of-lip-sync" and overall "warbling". I don't think what Jesse is talking about is the compression, as having heard this phenomenon before, the term "warbling" is a good choice. Now here's the bad news...I cannot remember for the life of me which card I had to replace! I do know it was one card in the DA20 at fault. Hopefully someone from Dolby will chime in soon to refresh my memory and enlighten us.
Posted by Jesse Skeen (Member # 586) on 02-05-2001, 09:46 PM:
Yes, the problem exists on most prints played in the auditorium, and they sound fine when moved to a different one. It is the only one in the place that has a DA-20 (the rest are CP-500s), so that may be where the problem is. I've only worked with one other DA-20 at another theater and that one worked pretty good. If anyone can confirm that it's probably a bad card then I can let our technician know.
Posted by Ari Nordström (Member # 288) on 02-12-2001, 01:30 PM:
I've never heard "warbling" on Dolby Digital with our CP500 and I must have screened a hundred different Dolby Digital prints by now. I have heard bad sound, however, badly mixed, badly recorded, badly handled, et cetera. I've heard Dolby Digital that sounds more like good old Dolby A, and Dolby Digital that, in essence, only uses the centre channel.And I've heard some excellent Dolby Digital, too. Crystal clear, well mixed, and well planned.
But warbled? Nope, not yet. I like Dolby Digital, and much prefer it to DTS.
Posted by Ken Jacquart (Member # 369) on 02-13-2001, 09:04 PM:
Jesse,You mentioned that you also have CP500's there... do you mean CP500D's (with the Dolby Digital cards instatlled)? If so, all cards in that section of the CP500D are identical to the DA20 except one. Your tech could then use these common cards for swapping, if necessary. Your description of warbling is interesting. There is a projector speed tolerance of +7 to -11% in the DA20... one could try swapping the Cat. No. 670 card (video preamp). I also wonder if you may be hearing 'block repeats'. If the DA20 cannot read one Digital block and cannot correct for it, then it will repeat the previous block up to 4 blocks in a row where it will revert to SR. There are other algorythm's which look for patterns of bad blocks which may not meet the 4-bad block rule. Anyway, block repeats may sound like a robotic type effect in the audio. If that is what you are hearing on a consistent basis, then perhaps something is up with your reader alignment (or the reader's light source). Your tech should be able to verify this with proper test equipment (an oscilloscope for video amplitude and DRAS for fun).
ken
Posted by Charles Everett (Member # 889) on 07-07-2001, 12:19 PM:
"Warbling" Dolby Digital? I've heard it happen at the UA Union Square in NYC. All I had to do was listen to the music."Yes, I'm Ready" by Barbara Mason sounded wobbly in Jesus' Son. Bjork's songs in the second half of Dancer in the Dark sounded wobbly. Both movies were in SRD/SR (no DTS, no SDDS).
Posted by Jesse Skeen (Member # 586) on 07-07-2001, 12:59 PM:
The problem I was talking about was fixed after our technician turned up the LED on the reader- in addition to the sound problem it was dropping out very often before he fixed it. This must be how it sounds if it's having trouble reading the soundtrack but not quite bad enough to drop out- I've heard complaints about other theaters that say it sounds like it's "skipping".