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Product Review - Circle Surround HTD1 Decoder - September, 1997

By Stacey Spears


Circle Surround Decoder
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Circle Surround HTD1 Home Theater Decoder; All Audio Connectors: Phono RCA jacks; 1 input pair (Left and Right) and 6 outputs for Left, Center, Right, Left Surround, Right Surround, and Subwoofer; Frequency Response: 20Hz - 20KHz +/- 1dB; Maximum Input Level: 12 V; Maximum Output Level: 12 V; Power Requirements: 120/230 VAC; THD: Typical 0.05%; Dynamic Range: 110dB; Sub Wooofer: 80Hz Low Pass; Input Impedance: 10 KOhms; Output Impedance: <150 Ohms; Power Requirements: 120/230 VAC; Power Connector: IBC; RSP Technologies, 2870 Technology Drive, Rochester Hills, MI 48309 (810) 853-3055 Fax: (810) 853-5937.


Over the past year or so, we have been hit with a couple of new multi-channel digital formats for home theater. Both Dolby Digital (DD) and DTS contain 6 discrete channels of "compressed" information that are NOT compatible with each other. I personally like both of the formats, and I have a decoder that does both, but not everyone has this or is willing to continually upgrade every six months to keep up. Of course there are only a handful of titles released in DTS, although there are numerous AC-3 titles (on both LD and DVD).

Stereo and Dolby Surround (DS) have been around for a LONG time, and there are thousands of titles out there recorded like this. ANY stereo or surround processor can play them. Pro Logic (DPL) has not changed since its introduction, but there have been improvements in the way of THX enhancements, which still use the DPL matrix. In the last couple of years, engineers have worked hard on further enhancements. Jim Fosgate is famous for his matrix technology, including his latest effort called 6-Axis. Lexicon is also keeping up with their Logic-7 matrix, but if you want their super matrix you must buy their processor. Well there is a new sheriff in town whose matrix is not only one of the best I have heard, but they are also licensing their matrix technology to the rest of the world.

This new killer matrix is, "Circle Surround" (CS). Perhaps you have heard of it? Well if not, you are in for a real treat! CS not only extracts true stereo rear surrounds, it also performs a great integration of the center channel with the left and right channels. They offer an encoder, though I did not have one on hand to encode 6-channel material into two channels. There have been several CD’s released on the market encoded in CS. My favorite part is that the processor is completely backward compatible with any matrix or stereo recording already out there!

While covering the actual encoding/decoding technology is WAY beyond the scope of this review, you can read their white paper which you can obtain by calling them directly, or printing it from their web page at (

The Little Processor that could

The HTD1 is a very basic no frills decoder, but let me qualify that by saying it is basic only in its features. The performance is far from basic. For $999.00 you get just the decoder, so you need a preamplifier. There is only one input pair: the left and right main. The processor has 6 outputs, which are the L/C/R/LS/RS/Sub. There are six pots on the front that let you adjust the output level of each channel on the processor. There is also a master volume control to match with your existing preamp and a cinema contour (roll-off) knob that lets you adjust for over-bright sound tracks (this is similar to the re-eq feature found on THX processors). There are 3 modes: Surround Video, Surround Music, and Stereo. You can turn the center channel on or off. You are also able to select the overall bandwidth of the surround channels, either 7k (similar to DPL) or 20k (full bandwidth). And finally you can control the steering of the surround channels by selecting Normal or Wide (super separation).

There is a built-in noise sequencer that provides standard pink noise for the 5 main channels, but it does not provide pink noise for the subwoofer. This is an area of debate. Some people say you cannot "properly" set the sub level with pink noise. Personally I believe it gives you a pretty good starting point, and is certainly better than nothing.

The processor is very easy to install, and the only part that will give you a good workout is setting the levels. The internal noise generator turns on and off very quickly, so you will have to run back and forth between your listening position and trim pots to set the levels, that is if you are using the Radio Shack SPL meter. Of course if you are equipped with the AudioControl SA-3050a, you can position your mic at the listening position and sit next to the processor while setting the levels, as I did.


I was blown away when I first installed the HTD1! The first thing I did was watch a movie, and I choose the CAV version of "Toy Story", primarily because it had the cool THX trailer at the beginning with split surrounds. The Circle Surround decoder did have a stereo effect in the rear, and it even panned to the left surround channel. I wish I had a CS encoder on hand to encode from the DD soundtrack into CS instead of comparing it to the DS version. Nonetheless, the effect was very cool, and I enjoyed the movie. I next watched "Jurassic Park", and this was also enjoyable, but there were a few instances where there appeared to be "pumping" in the front channels primarily to the right side. (Pumping is where the sound moves from the center to the side speakers when it is not supposed to.) This only happened for a little bit, then went away.

I watched several other films, and another effect I noticed was a lot of dialog being sent to the surround channels. While I like the stereo surrounds, having dialog bleeding into the surrounds bothered me. It felt like it was wrapping the front channels around to the side and rear. This is definitely an artifact, but keep in mind that these were a set of two channel stereo soundtracks being decoded into four channels of stereo front/rear surround. The best part of the CS to me was the effect it had on the overall soundstage. It remained very wide, and not once did it collapse to just a center channel, as compared to regular DPL which can collapse during dialog-intensive scenes. One of my favorite examples of the DPL soundstage collapsing is on, "My Cousin Vinny." During the sequence when Joe Pesci and Marisa Tomei are driving into town for the first time, as they are turning corners, the music keeps going from center only to full stereo and back to the center again. With the CS in place, the whole presentation remains wide and very pleasing, not at all distracting like DPL is. The only downside I noticed to CS on movies that are encoded in DPL is the dialog that leaks into the surround channels and the occasional pumping in the front channels.

As good as CS was on movies, it did not even prepare me for the effect it would have on music! CS is much more impressive on music than it is on movies, at least the ones encoded in stereo or DPL. Every CD I threw at it was much more enjoyable than listening to it in standard stereo. CS does a fantastic job of blending the center channel with the left and right. It also spreads the entire performance out beyond the left and right speakers, and it wraps around you with the aid of the surround channels. During music, I never noticed the vocals coming out of the surround channels like the dialog did on film.

I had the Junior Wells CD from Telarc recorded in both DTS and CS. The two recordings were VERY close, and the CS was able to duplicate the DTS presentation almost to a T. I have to give the overall presentation to the DTS version, as it was cleaner and less edgy. But I was not disappointed with the CS recording, and in fact, if 5.1 CDs did not exist, I would be completely happy with Circle Surround! A list of CDs encoded for Circle Surround (as well as CDs encoded with other surround sound formats) can be found at


Discrete technology is available today, and the only problem is that they are not compatible with each other. I own a hundred or so CDs that benefit from the technology of Circle Surround, and I would not even try to replace them if a discrete version of each CD were to become available. As a technology, Circle Surround is Fantastic, and I hope several other manufactures will incorporate it into their Pro Logic/AC-3/DTS processors.

The HTD1 decoder by itself is rather limited, since it only does CS decoding. There is no remote control functionality, and it requires a preamplifier (which is OK if your preamp has a remote control). The HTD1 would mate very well with the Millennium DTS decoder and a preamplifier (the Millenium throughputs other outboard decoder signals automatically), to give you excellent matrix and discrete technology. If you get a chance, I recommend you listen to the HTD1 and Circle Surround.

Stacey Spears

Copyright 1997 Secrets of Home Theater & High Fidelity
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