Yesterday, I wrote about David Robinson’s affection for HDTT (High Definition Tape Transfers) DSD 128 digital files. The company acquires non-copyrighted consumer analog tapes and then makes very good digital copies of them to both PCM and now DSD. They used to feature HD-PCM files at 192 and 96 kHz sample rates but have recently added DSD 64 and DSD 128 versions to their offerings. The files can be downloaded or purchased on data discs.
Having spent a great deal of my professional life working with analog tape (both multitrack AND stereo), I was curious whether HDTT had any free samples to check out. They have a number of items on their site and I downloaded the Largo movement from J. S. Bach’s Concerto in A minor at both 96 and 192 kHz 24-bit PCM.
There wasn’t any information on the site regarding the speed and format of the original master tape. It could have been half track stereo or even quarter track stereo (although I doubt it) at 3 3/4 or 7 1/2 ips…maybe 15 ips. There weren’t a lot of 15 ips machines around back in the 60s. I know my own Wollensak unit was limited to the lower pair of speeds. This would mean that the highest reasonable frequencies that I would expect to see or hear on these transferred files would be around 20-25 kHz.
The dynamic range of a consumer tape from 50 years ago would also be less than ideal. Without Dolby noise reduction, one might expect to have 60-65 dB or dynamic range on the original master. Subsequent copies, however, would be reduced by 6 dB at each analog transfer stage. The normal process for an analog tape would be: record the original master and then edit it after making a safety copy, copy the edited original through mastering (the first transfer stage) and then make a dub master (second copy and another 6 dB lost) from which customer copies are made (third copy stage). The best we can hope for is that a terrific master analog tape was made and we get a digital transfer that has 18 dB more noise than the original (as a result of the three generations of transfers. Remember that this is equivalent to about 8-bits of dynamic range in PCM digital…not matter what size the digital words used during the transfers.
At the end of this process we have a sound that David Robinson of Positive Feedback Online said, “…moved me to my soul”.
I listened to the Bach sample in my state-of-the-art studio (using a Benchmark DAC2, Bryston 4B power and B&W 801 Series III speakers and I experienced something considerably less moving. The lack of dynamic range was the first thing that I noticed. The sound was nice enough although to my ears distant, detached and flat sounding with very little depth and timbral accuracy (the low end was thin). I would suggest that you download the AIX Records Bach Brandenburg demo recording that I placed on my FTP site (which can be acquired for evaluation without charge) and then download the Bach from HDTT. Compare them and let me know what you think. Which, if any, moves YOUR soul?
Figure 1 – Spectragraph of an HDTT download of a Bach Concerto movement. Click to enlarge.
So here’s the spectragraph. You’ll notice that the 192 and 96 kHz versions are identical up to 40 kHz. After that the 192 version is devoid of any musical material and contributes nothing to the overall sound. The measured dynamic range is about 50 dB or well within 9-bits of PCM equivalent (just past the dynamic range in the 8-bit games we used to play with our Creative Labs Sound Blaster PC sound cards). For comparison, the AIX Bach recording topped out at 97 dB or 17 bits! I can understand the attraction if a performance was historically important or by a prominent conductor but as a model of what we should be striving for sonically…I don’t get it. There’s got to be extra musical reasons behind a reviewer lauding a track that pales in comparison to a contemporary HD-Audio recording. And I haven’t even started talking about a 5.1 surround mix!
Take a listen and please let me know what YOU think.