Analog 6-channel Home Theatre Audio Project
Straight out of the junkbox and into the livingroom
Fred M. Niell, III
The Analog Surround Processor
In this age of Dolby Digital and dts decoding standard on recievers under about $200, you might wonder why I decided to do this project at all. Two reasons: 1) at the time I was a graduate student in physics, and therefore had no money to buy a $200 reciever, and 2) I am a strong believer in analog electronics when it comes to audio. Since I can't afford to purchase the single-chip surround solutions available from a few manufacturers, (and for that matter can't afford to buy new components) I decided to build the entire system from my junk box, with the exception of a few potentiometers from Radio Shack. While my CD player is indeed better at accurately reproducing sound than many LP's, I just can't give up the panache of analog sound reproduction. So, I set about to create a Dolby Pro Logic surround sound style system without the aid of digital wizzardry using junk parts.
I started at the Dolby Labs web site, downloading their "white paper" documents on their Pro Logic system. While there are no schematics available, there is extensive documentation on the theory of their system [Figure 1]. The Pro Logic system takes the two channel output from a stereo source, and decodes it into 5 or 6 discreet channels of sound, depending on the sophistication of the system. Typically, the signals from the stereo source are digitized, and the math and delays are done with a dedicated processor. The digital signals are then converted back to analog, and sent to the amplifiers.
Clearly, getting all the functions done using analog trechniques was not going to be easy. In particular, the delay was going to prove difficult. Their "steering" matrix system (at the time I was researching it) was not well-described, so I had to improvise my own solution. More recently, it has become clear that I chose a system called "gain riding," as noted on the Dolby home page.
How it works
The schematic is shown in figures (2,3,...). I found that designing the system in parts was helpful, so the schematic is presented as such. The system is broken down into the front end, the delay and noise circuit, the steering matrix and amplifers, and the power amplifiers.
The front end is the heart of the surround system. The input utilizes the Haefler matrix idea, but extends it a bit farther. To begin, we have a gain 1 buffer amplifer for each channel. The signals are then level-shifted and sent to input buffers. The Right (Pre-InR) and Left (Pre-InL) signals are summed and differenced with standard op-amp circuits. The resulting InR - InL signal is subtracted off of the Pre-InR and Pre-InL, giving the post line-outs. The line-outs go to gain 1 buffers with 100 ohm output impedance. The summed Pre-InR and Pre-InL are fed to the center channel output buffer and filter, and the subwoofer filters. The subwoofer filter is a 9-pole low-pass filter set at 200 Hz, with an overall gain of 1. The center channel filter is a two-pole high pass set at 800 Hz. The front end was designed taking into account the smallest details to reduce noise and hum.
Devising a delay system proved to be difficult. I spent a lot of time going down blind alleys with barrier bucket designs and actual long-delay cable. However, I remembered having seen a Fisher tube amp from the 60's that utilized a "spatializer." The spatializer consisted of a driver and reciever circuit for a spring reverb unit. Since I had recently torn down a Peavy amp for parts, I had a medium-delay spring reverb laying around. I used a bandwidth-limited driver stage, consisting of a bandpass filter (400-6000 Hz) and an LM386. From the reciever stage, the reverb signal is summed with the original InR - InL in a simple mixer. The sum signal is sent to a National Semiconductor DNR noise reduction chip. The noise limited signal is sent on to the steering section.
To accomplish the task of replicating the "steering" effect, I decided to use active integrators and ADI voltage controlled amplifiers (VCAs). The Pre-In signals are integrated with a time constant on the order of 200ms. The integrated signals are then differenced. The difference signal is passively integrated with a time constant on the order of 2.5s. This signal is called the steering signal, (Vst). Vst is buffered and split; one output is for the right VCA and the other is inverted to create the left VCA control signal. The noise limited surround signal is split and fed to the VCAs. The VCAs, in turn, provide opposite gain responses according to the time integrated PreIn signals. These become the right and left rear surround channels.
The power amplifiers used in my prototype are simply National Semiconductor LM3886 chips, using their datasheet example.
It is useless to write a story about an electronics project and not include schematics. The entire scheme is relatively easily broken up into five separate divisions. Below there are five separate pages of schematics. The first schematic page has the input buffers, summing amp, and subwoofer filters. The second schematic page has the active integrators and steering signal generators. Third is the left-right steering amplifiers. The fourth schematic page has the delay generator. And the fifth page has the National Semiconductor Dynamic Noise Reduction chip.
|Here's the input buffer schematics. On the left are the input buffers. Below is the summing amplifier to generate the basis of the center channel and subwoofer channel. There is a 6 pole filter to 200Hz as described above. There's a 600 ohm output buffer for the sub channel.|
|The second page includes the active integrators and steering signals for the voltage-control amplifiers. On the top left there's high precision integrators followed by a difference amplifier. Then there are buffers and an inverter providing the steering signal. Note that there is a gain trimpot on the steering amp that controls the gain of both left and right steering channels.|
|The third page is where the magic happens. The steering signal is fed into the voltage controlled ampliciers (VCAs). The VCAs are fed with the same common summed surround signal, but inverse steering signals. This results in most or all of the surround signal being steered to the left or right surround output channel. Also note that the VCAs have current outputs. Thus, the LF356s are used as current-to-voltage converters. There is a little bit of HF filtering as well.|
|The fourth page includes the delay generator. Notice the 7.5V generator. This was a little bit of poor planning. I didn't realize the LM386 had ground-referenced inputs. So, I reverenced them to 1/2 the supply voltage of the amp. But, I fixed this with an RC filter, as seen in the schematic. This is also where the "spatializer" idea comes into play. The medium-delay reverb tank supplys the required delay, and the delay mix pot allows trimming of the output delay balance.|
|The fifth and final page includes the DNR chip providing 2-channel noise reduction to the processing chain. Most importantly, the DNR chip provides the return from the delay generator with much-needed noise reduction. For some reason the return on the reverb tank is quite noisy. Also, the center channel can be a little noisy. The high slew-rate opamps can generate some rather hashy signals (Lin and Rin sum is noisy).|
Building the Analog Surround Processor
In low-level audio projects, it is important to keep leads short, and grounds common. However, since I am a physicist, I like to "shoot from the hip," and opted to use breadboard, twisted pair interconnects, and basically the "rat's nest approach." Were I so inclined, I could have easily printed up the PC board included in my design. However, since the maze of wires hasn't been touched with easily thousands of hours of operation, I haven't gone about making the board.
As with any analog system, the design is rife with gain settings. I have tried to make it hard to ruin anything by setting the gain too high in any one spot. Generally, the audio signals are kept in the 1-1.5 Vpp range. The VCA control signals are in the few volt range, but they are for all intents and purposes slow moving DC. The mixer control, surround gain, subwoofer gain, and master volume are all put on the front panel. This allows for easy access to the relevant controls. In hindsight, it would have been useful to have some others on the front as well. For example, the gain of the Vst buffer is what I liked to call the "sea sickness" control. If it was set too high, the resultant panning would be enough to make you dizzy. That said, once I arrived at a useful setting, I didn't change it for over a year and a half.
You may think I have an exceptionally well-stocked junk box. That is perhaps the understatement of the century- and my wife can attest. Suffice it to say, most of the components are readily available. If they are not, just substitute the component you don't have for something similar. That is the beauty of analog design; flexibility.
Notice that I have used a number of LF365Hs. These are not exactly readily available amp chips, but they found their way into my junkbox over the years. Also, I have used some rather hard-to-come-by analog chips (such as the quad VCA and the DNR chip). These are actually not as hard to find as you might think. The LM1894 is still available for samples, and the SSM2164 is also available via sample request. This is also something quite nice about the design. The VCAs and the opamps used are not critical. The DNR chip is the only one that doesn't have a readily available replacement. However, this chip is still used in commercial electronics on a regular basis, and its supply is not exactly threatened.
It is also important to mention that this was wired point-to-point on several boards. This is not the recommended construction method for any analog project. It took many nights of troubleshooting to remove the ground loops and re-solder connections.
Using your Analog Surround Processor
Start off by disconnecting the power amplifiers. Getting the signals right before overdriving and possibly destroying your amplifiers is key. Begin by feeding the stereo source into the input, and trace the audio through the system. The signal shouldn't get more than a little amplified, and at no point shuold it exceed more than 1.5 Vpp. The National Semiconductor noise chip and the LM386 will complain with too much amplitude.
Set up your speakers as you would with a digital home theatre setup. Two mains in front, a center speaker, a subwoofer, a left rear surround and a right rear surround.
Since there aren't really any rules for this thing's operation- simply fiddle with the controls until the signals are all within reasonable limits. Then listen. Then fiddle more. For adjustment of the Vst, I suggest trying out a CD you know has great stereo effects. You can play a track over and over again until it sounds about right.
Once you have the gross adjustments finished, you can close the box up, and start listening to CDs and movies. The reverb mix control allows you to have a continuum of echo effects, from a concert hall to a recital room. The other gain controls are relatively intuitive.
Your system, when tweaked properly, will sound like a Dolby Pro Logic system. You will probably need a fair amount of time to get it sounding just right. Once you have it perfected, though, you will have learned a ton about analog electronics design, and what's more- you will have a great sounding home theatre system that cost near nothing!