I recently got a Sony TCD3 (WM D3) for recording, and I've noticed it records and plays Dolby B much better than my old Technics deck and even my WM-Ex670. Shouldn't they all decode about the same? Why does my 670 sound so muffled compared to the D3? Does an amorphous head really make that much of a difference?
@Impala67 An "older Technics" deck can also suffer from head wear, even if it was TOTL (they did use higher-quality heads). It may also have an azimuth alignment issue, and the same applies to the Sony WM-EX670. Amorphous heads, however, offer significantly greater durability. In addition, not all Dolby chips—or the circuits built around them—are created equal, so that results can vary. As @TooCooL4 correctly pointed out, these chips live in aging equipment, and the surrounding capacitors and resistors often have a much greater impact on the sound than the IC itself. In many older units, component drift means they no longer meet their original specifications. This is one of the reasons I stopped using Dolby NR.
@Impala67 It's not the amorphous head that makes the difference, but the tape path alignment and level + bias adjustments (latter only if the tape was recorded on D3). For one, playing back a tape recorded on the same unit will effectively cancel out any minor tape path misaligment as it will be the same for both PB and REC. For optimal Dolby decoding you need 2 things: 1. A deck with external manual or auto calibration, so you can calibrate level and bias on the individual formulation you are recording on. 2. A player with correct tape path alignment and correct playback levels. Units like EX670 have no adjustments in regard to tape path or levels. While generally levels are correct on such units, tape path misalignment caused by roller/capstan issues are pretty common. Yes head wear can be a factor, but has to be very significant (like roloff from 7-8kHz) to throw off Dolby decoding, especially on Dolby B (C si way more sensitive). While Dolby circuits may not all be equal, the components with critical values always have very low tolerances and long-term stability. The real problem on units with adjustable Dolby levels is incorrect adjustment from factory rather than component drift. It's why for many "Dolby never worked". Some early units which were calibrated at factory with incorrectly recorded DIN calibration tapes had the levels off from the beginning. It's an issue that has been documented on tapeheads. At the end of day, of course, it's simpler to never use Dolby NR and many people do exactly that. This way some minor tape path misalignments won't be very audible and many probably won't even notice it (despite it is there). But the same misalignments are amplified by Dolby, making them obvious (muffled sound or breathing artifacts). In practice, even very early units can be made to encode/decode Dolby perfectly. However some will work way better in as-is condition than others.
