lionel@eludom.DEC (Steve Lionel) (08/26/85)
With more and more stereo broadcasts in my area, I was itching to get an MTS decoder for my VCR (Sony SL-2700), as it's what I use as a tuner. However, I was discouraged by the reviews of existing decoders pointing out that they did not achieve the performance levels which the MTS encoding permits. The Zenith decoder has reveived a lot of complaints, and the Sony decoder would make me pay extra for a small amplifier I'd never use. So it was with great delight that I read the review in Stereo Review of the Proton 690 MTS decoder. As I am a strong believer in Proton products, owning a 600M monitor, I was pleased to see that the Proton decoder performed much better than others on the market, especially with regards to channel separation. So, I went to my nearby Proton dealer and ordered a 690. I was told it would be "7-10 days", but after four weeks, I looked elsewhere and found it (in stock!) at another store for $20 less. My eventual price was $129.99. (Compare to $175 for the Sony MLV-1100 and $100 for the NEC decoder.) Eagerly, I unpacked it and hooked it up to my VCR. As the Proton had a level control, I did not anticipate compatibility problems. Wrongo... The MPX output of my VCR was far too low for the Proton decoder, and even turning the level adjustment full-up didn't light the stereo pilot. The audio level was also much lower than it should have been. (I later discovered that the Sony "standard" level for MPX output is 126mv, while the Proton decoder wants 500!) Sadly, I packed the decoder to return it to the store, muttering about having to settle for a more expensive, but not as good decoder. It was then that I thought to dig out my SL-2700 service manuals, which I had ordered a while back. Very quickly, I discovered that the MPX signal output level is adjustable, via a trimmer on a circuit board. So, I unpacked the decoder, hooked it back up again, popped the cover on the VCR and tweaked the trimmer. Voila! As I turned the trimmer, the volume increased and then the stereo pilot lamp came on! I was in business! So, the moral of this story is that when you go to mix-and-match MTS decoders, you must be aware that different manufacturers use different MPX signal levels in their equipment. The NEC decoder has both "high-level" and "low-level" inputs, so perhaps it would have worked with my Sony VCR directly. For those of you who run into this problem with the Proton decoder, or other, look to see if the MPX signal level on your VCR/tuner is adjustable. On the SL-2700, it's a small trimpot in the center of the top right-hand board, marked "MPX ADJ". Using a very small screwdriver through the hole in the PC board, turn the trimmer to the left to increase the signal. You want to get it so that the pilot just lights reliably when the decoder's level adjustment is at the center detent. Too high, and you'll get distortion. For those of you who are interested, the Proton 690 MTS decoder is the first I've seen that publishes its specs. I've listed them below. With the 690, it will automatically select stereo when a signal is present, otherwise it will send mono to both outputs. SAP and "both" (mixed L+R in one channel, SAP in the other) are other options. Front panel LEDs tell you whether stereo and/or SAP signals are present. Proton 690 Specifications: TV MPX Input Level MIN 500mV Rated output level 700mV Stereo separation 30db (at 1kHz) T.H.D. Mono: 0.1% Stereo: 0.3% SAP: 0.6% Frequency response Stereo: 50Hz-10kHz +-1.5dB SAP: 50Hz-8kHz +-2dB S/N Ratio Mono: 75dB Stereo: 65dB SAP: 70dB Steve Lionel