I remember reading this years ago at 'Tascam Forums' site and include it here as a point of reference:
Originally by Tascamwiz:
'I have worked on various brands of analog tape machines since the 80's.....Studer, MCI, Ampex, Tascam, Fostex, etc, as both a technician and recording engineer. I sold my last Ampex ATR in the mid 90's and have been all digital since......well......in the past few years a lot of my associates have been digging up analog machines in various states of neglect and dis-repair and bringing them to me to resurrect (as if I have nothing better to do).
I'm not sure if anyone has gone into this recently, but, I have noticed among the narrow format machines that almost every single Fostex G-series, Tascam MSR, TSR, 238, etc.... I have worked on that had Dolby-S noise reduction has had problems with the Dolby-S chips. I have seen them burned, shorted, and open. Some machines have 1 or 2 dead NR channels, some are nearly all dead. It appears to me that these NR chips are very sensitive to both drifting circuit tolerances and power supply voltage tolerances. I have seen these issues on several different brands of reel-to-reel and cassette recorders, so I do not think it is related to poor product design. These chips have not been made for several years and are not available as individual replacement parts when they do fail.
After talking with some colleagues, I discovered that the probable reason for these issues is that due to cost constraints the original Dolby-S circuit design (simplified and derived from Dolby-SR) had to be condensed down to a single asic in order to meet integration cost targets with the various manufacturers of consumer audio tape products in the early 90's. Large scale integrated circuits were still fairly new in the late 80's/early 90's and the Dolby-S asic chips were made using a new (for 1991) fab process known to have problems. It is called the bipolar master slice (M/S) process. This fab method had several "advantages" over a typical custom IC design, including lower cost, fast turn around time, more design flexibility, and supposed quick assessment and correction of design problems. The advantages of the new M/S fab method were key to the design of the S-type IC due to lower cost requirements. This is because circuit design changes to accommodate integration and performance requirements, and testing and evaluation was difficult. The resulting IC was a low-cost, single package device that was readily accepted by manufacturers for low cost and easy integration into analog products as digital products became more and more mainstream.
It is possible that these problems may have been resolved later on, but, since the majority of Tascam and Fostex machines that were equipped with Dolby-S were made in the early 90's, it is fairly certain that Dolby-S problems with these machines will be fairly common as these machines get older.
Aside from technical problems with Dolby-S, I see machines advertised all the time as having the "desirable" Dolby-S NR vs the "undesirable" DBX NR. To me this is/was pure marketing BS and is something I do not understand because Dolby-S NR IS NOT the same thing as Dolby-SR in terms of capability, complexity, and quality. Dolby-S is/was a simplified, cut down, and cheaper version of SR developed primarily for the limited audio bandwidth and simplicity of cassette recorders. Any cassette deck manufacturer that wanted to incorporate Dolby-S into a product was required by license to include an automatic calibration routine as far as setting record bias/eq/level for a particular tape was concerned. This is because Dolby-S is extremely sensitive to mistracking due to level and frequency calibration problems typically as a result of incorrect bias/eq/level settings. The fact that it was adapted to be used on machines that record in a studio is mainly in my opinion because Fostex used Dolby because Tascam used DBX, and, Tascam later used Dolby because Fostex used Dolby......marketing and sales, nothing more.
If calibrated correctly by a skilled tech, Dolby-S gives at most 3db of NR improvement over Dolby-C with a bit of low frequency NR as well and good sound quality.......as long as the tape and calibration does not change!!!
Most narrow format Dolby-S reel machines have dozens of circuit cards and adjustment settings that must be maintained on a regular basis in order to maintain decent fidelity and correct NR tracking. Pro reel machines obviously no not have an "auto-calibration" button like some cassette decks do, so Dolby-S NR mistracking is quite common. Aside from the ASIC problems with the Dolby-S chip itself, every single Dolby-S machine I have worked on had at least several channels in which the calibration was way off and with subsequent poor sound quality with the NR switched in.
DBX NR on the other hand is a relatively simple 2:1 linear companding NR system that allows for a lot more signal on tape before saturation, 120db of dynamic range, and 95+db s/n ratio. I have heard of all the reports of DBX "breathing" and "pumping" and other audible artifacts. The Tascam MSR generation of tape machines has in my opinion the best integration of DBX of any analog tape device ever made. As long as the tape machine is designed to have flat frequency response, DBX NR can provide amazing sound quality and low noise with zero artifacts, and, since it is not level sensitive, will tolerate a fair amount of mis-calibration before audible artifacts occur.
I have 2 MSR-24's on my bench right now, each with freshly lapped heads, a complete tune-up and aligned to RMGI 911 tape. One has Dolby-S, the other is DBX. I recorded a couple songs from a Mobile Fidelity SACD of Patricia Barber's "Cafe Blue" to tracks 8 & 17 (stereo L&R) on both of them and fed the outputs through a Sonic Frontiers preamp and a pair of Balanced Audio Technology VK-150SE monoblock tube amplifiers driving a pair of Magnepan MG20.1 planar loudspeakers......about $25,000 worth of playback equipment not including the Tascam units. I then did an AB comparison between the SACD player and the MSR-24 with Dolby-S and between the SACD player and the MSR-24 with DBX for 5 different people. They could better than 8 out of 10 tell the Dolby-S MSR-24 copy from the SACD source. On the DBX MSR-24 it fell to about 50/50 which is statistically significant in that they could not tell and were guessing.
So.........my 2 cents worth is if you are looking for a narrow format tape machine, the Tascam TSR-8 and MSR-24 with DBX are hands down the best you can get in terms of sound quality and reliability......the MSR-16 might be good to, but, its track width is really getting down there. I would not pay extra for Dolby-S especially given the problems with the ic's.'
