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Mix Magazine

This installment of The Bitstream column appeared in the March 2002 issue of Mix Magazine.

The Bitstream

This column discusses discusses data rates and storage requirements…

Cramped Quarters

This month, I’m forgoing my speculative fiction to comply with a reader’s request for some practicalities. Let’s look at “speeds & feeds,” the numbers associated with a product’s specifications and performance. Specifically, data rate versus word length, how much space high density audio really consumes and why uncompressed audio just won’t fit down a typical ADSL pipe.

Back in the first months of 1998, I worked up some numbers for “high density” audio production. That spreadsheet has evolved over time but pales in comparison to Bob Stuart’s MLP worksheet. Along with Bruce Nazarian’s DVD-V bit budget spread, I can get a sense of what’s possible when planning most any CD or DVD title.

Why should you give a rodent’s posterior about all this? Because, as my hirsute editor said, “We live in a hybrid world…,” where consumers have widely varying acquisition and playback capabilities. If you want your own or your client’s stuff available to the biggest market, you need to consider storage and distribution requirements for that data.

To start with a simple case, take the CD format: 44.1 kHz times 16 bits times 2 channels results in a data rate of 1.4112 Mbit/second (Mbps) or 0.1764 MBytes/second. 60 minutes of that stuff needs 635.04 MB of raw storage space, excluding overhead. Nothing too scary there…

Now let’s take a look at some slightly more involved numbers:

Chan.
44.1 kHz 48 kHz 96 kHz 192 kHz
2
793.8 MB 864 MB 1,728 MB 3,456 MB
3
1,190.7 MB 1,296 MB 2,592 MB  
5
1,984.5 MB 2,160 MB 4,320 MB  

This table represents raw storage requirements for 20 bit audio, in Megabytes, again excluding overhead. A 20 bit word is a good compromise for music distribution since redithering a 24 bit final signal down to 20 bits saves a bunch of space, 17% to be exact. Not to mention the data rate reduction, which I’ll get to in a minute. 20 bits also preserves sufficient dynamic range for even the quietest home playback environments, which are often much quieter than your studio.

I didn’t include more than two channels of 192 k since only DVD-A supports that file type and only two channels worth at best. Score one for SACD. The same table with 24 bit files is a tad fatter which, you may think, is why we have DVD: all that extra room to store long words. But no, zat eez not zee case, my friend. HDCD works just fine to encode extra dynamic range on a CD, yet it failed in the marketplace (if you call getting bought by Microsoft a failure - OMas). No, DVD was created to provide an all singing, all dancing replacement for the aging CD format and most members of the consortium could care less about fidelity. Fidelity doesn’t sell.

The mighty DVD format labors under many serious limitations, one of which is the sustained data throughput rate of the transport. To play your basic 96/24 five channel material, you’ll need an 11.52 Mbps pipe, way too big for DVD and pushing your luck for 100BaseT, I might add. To the rescue comes lossless data compression in the form of Dolby’s Meridian Lossless Packing. An MLP–packed version of worst case material, what Meridian classifies as “Metal,” would be only 7.18 Mbps on average.

MLP inspects multiple parameters when performing it’s magic, including dynamic range and spectral spread. So, a 96/24 five channel solo piano track would, due to it’s wider dynamic range and statistically limited spectral composition, pack down to an average data rate of only 4.4 Mbps. That’s within most any modern pipe’s capacity.

Of course, DVD has other tricks to reduce data rate and storage space, like half–rate groups for rear channels and LFE, along with better downmix abilities than DVD-V. Since listeners that have stereo playback would apparently be less interested in spatialization, one could safely assume that, for multichannel material played back in a two channel environment, tonal balance is the important issue when downmixing. Given that, MLP’s downmix abilities should suffice for the majority of cases, eliminating the need for a separate LtRt mix, in the Audio zone anyway. MLP also elimintaes the need to redither 24 bit material to 16 or 20 bits, since it “knows” how to eliminate any unused dynamic range in the data.

Earlier, I threw out some numbers with the caveat, “…excluding overhead.” This brings me back to storage issues and that overhead. If you dig around in DVD-Audio bit budget calculations, you’ll find a better than 50% discrepancy in the storage overhead required between PCM and packed files. The difference turns out to be related to DVD sector formatting. Bob Stuart at Meridian Audio helped to create MLP and was kind enough to explain that, “…because MLP carries decoder instructions very efficiently in the stream, it is possible to use less overhead in the headers of each disc sector than is possible in the LPCM case…In fact, MLP is the most efficient stream in the DVD family. Moving video is less efficient than LPCM and the lossy-compressed stream options (e.g. MPEG, DTS, Dolby Digital) are also packed less tightly on the disc.” MLP has better fidelity as well, I might add.

Now, this is all fine and good for physical media: format your disc, burn it and watch ‘em march out the door into the open arms of Jane Q. Public. But what I think is more interesting is the data rate required to push these files through a pipe. Maybe you’re sending a mix out for approval via the public network. Or, maybe you work in a broadcast environment, where folks think nothing of playing audio over a LAN. I know…as Marvin might say in this situation, “Don’t talk to me about jitter.” Ethernet, 1394 and most other LAN-like mechanisms exhibit absurd amounts of jitter but that’s not important in this discussion and, yes, jitter reduction can be achieved if needed. The point is that the aforementioned 1.4 MB/sec. rate for CD quality material can easily travel uninterrupted down a 10BaseT connection if you have the right hardware. Increasing the word length to 24 bits brings that rate up to 2.1 MB/second, making things more difficult for low speed Ethernet. Of course, Fast Ethernet would ensure uninterrupted service but what about trying to move that data over a “broadband” connection?

I don’t know about you, but my ADSL service is the cheap version, typically providing 384 Kbps, topping out at about 512 kbps and only in the download direction at that. The alternative is a download to disk approach, which works great as long as the number of channels and duration remains low. As my colleagues and I have found, it’s painful even with ADSL or cable to move multichannel, high density files around on the public network. An alternative is SDSL service, which should come with a Service Level Agreement or SLA, guaranteeing some minimum QoS and monetary compensation for outages, if you can afford it.

Another desirable feature for which you pay extra is a fixed IP address. This means that your local host can be addressed from a remote location, say, your client’s place. This assumes that you’ve configured your machine for insecure but convenient FTP access ahead of time. Consumer broadband accounts sometimes offer fixed IP addresses as a value–add but usually your router, gateway or modem is assigned an IP address dynamically, making it tough to FTP in to your local network.

So, there you have it. My 512 Kbps “broadband” pipe can really only carry about 5 bits of 44.1 stereo audio, not too hi–fi by anyone’s standard. So, I continue to groove to the dulcet strains of lossy codecs, the subject of next month’s Bitstream. Until then, stay warm!

Bio

OMas has just returned from a long weekend of winter camping on the brawny shoulders of majestic Mt. Whitney. He took his iPod, so this column was created while under the influence of 192 kbps VBR stereo MP3s.

Thanks to George Blood for suggesting this column’s subject. Got a topic? Give a shout and I may write it up…For links, back issues and occasional commentary on The Bitstream.