Richard White goes all cryptic with One Thing's stereo decoder.
The Leak Troughline tuner is excellent; the stereo decoder is not. Solve this problem and any high-quality mono tuner (Leak; Rogers; Pye; Quad) can get a whole new lease of life. Not only but also, many stereo tuners have 'standard' decoders fitted almost as an afterthought. These too can benefit hugely from a better-specified modern decoder. One Thing's Stereo Demodulator sustains an honourable line and is available in an outboard version as illustrated, or as a 'factory-fitted' upgrade to your own tuner.
For newcomers to the tuner scene, multiplex is a method for combining two or more signals in such a way that they can be unravelled again when required. The technique was developed as being the most retro-compatible when FM stereo broadcasting standards were set years ago.
A simple analogy (fig.1) shows the basis of the idea. So long as the two sets of points are correctly synchronized, the two groups of trucks can be sent along the single line alternately and re-switched at the other end with no-one any the worse off. A great part of the secret of successful operation is making sure that the points switch at the right time: if this gets out of synch, it is easy to see that the separation which is the object of the exercise might degenerate into an unseemly mixture or a series of pile-ups, resulting in no signal/trucks getting through at all.
Whatever refinements are adopted, the 'switch' synchronization is governed
by a pilot tone transmitted (by the broadcaster) with the encod-
ed stereo signal.
Because the basic requirements were decided long ago, modern-day decoder designers are more concerned with ensuring that the technology gives the best possible performance within the given limits: low noise is an important factor, as is stability of separation.
One Thing's decoder is unusual inasmuch as it does not amplify the incoming
signal before decoding it, so the overall gain is less than unity (or 1x).
The manufacturer says that this hasn't been an issue so far: most tuners are
capable of pushing out the requisite 320mV multiplex signal without needing
an additional amplifying stage and the effect that can have on the sound.
If there is insufficient signal for decent stereo, the beacon fails to light
and you have the options of switching to mono for lower noise or maybe using
the handy Blend control to pick an optimum in-between point.
Tuning in to Radio 3 for what is probably the least meddled-with signal on
the dial, I was astonished at how good the broadcast was.
Everything from chamber works to full orchestra to plays to news leapt into
life. A gloriously rich spread of crisp, clear stereo sound wafted forth whichever
way the dial was pointed. Even the ultra-compressed commercial stations sounded
vaguely human for a change.
In use, I found that I simply forgot about the One Thing - ungrateful I know! My admiration tended to settle on the tuner and (gasp!) the programmes. Given the steady improvements in stereo broadcasting over the last 30 years, I don't think my tuner has sounded better in its life.
ONE THING DEMODULATOR
This decoder had accurate 50pS de-emphasis, so frequency response was flat and extended to 14kHz (-1 dB), as our analysis shows. It will have a good balance to its sound quality as a result. Channel separation was high at 45dB in the mid-band and 31dB at 1OkHz; these are very good figures. Distortion measured a miniscule 0.1 2% and noise was absolutely minimal. However, the excellent noise performance probably resulted from the lack of gain before the decoder chip, which in turn made it insensitive. A minimum of 300mV in i s needed to trigger the stereo beacon, yet maximum input was 800mV, before overload distortion set in. The makers state most tuners offer more than 300mV MPX out but in my experience MPX out signals at HI-FI WORLD can be quite weak - 50mV or so. Perhaps my experiences arc unrepresentative Whatever, it would help if this unit had under/over lights, a small amount of variable gain or some other way of coping. Users otherwise are best advised to measure output from their tuner to ensure compatibility. This matter apart the One Thing measured very well and will do justice to high quality tuners. NK
| Frequency response | 2Hz-14kHz |
| Stereo sepeeration | 45dB |
| Distortion | 0.1 2% |
| Hiss (CCIR) | -78dB |
|
Trigger level |
300mV |
| Max input | 800rnV |
WORLD VERDICT
OUTSTANDING - Superb sound, something we'd use ourselves.
This article first appeared in HiFi World April 1999
