Friday, September 30, 2011

Surround Sound Decoder


The surround decoder is based on the "Hafler" principle, first discovered by David Hafler in the course of the 1970s. The original idea was to connect a pair of speakers as shown in Figure 1, for use as rear speakers in the surround setup.

It is fine as it is, but the problems are created when the main speakers are bi-amplification or transitional assistance, as it is a sign of full range output / total available for the rear speakers. There is no way to control the playback level, because it always ends up being the difference signal between left and right.

If the signal is mono, the signal on both channels is always more or less the same, and should not be starting from the rear speakers at all.
 The Original "Hafler" Surround-Sound Matrix

This circuit works by allowing the rear speakers to play only the difference signal between left and right outputs. All stereo encoded material has a certain difference between left and right (otherwise it would be mono), and this difference is that the output signal from the rear speakers.

It is important to ensure that the connection between the rear speaker terminals are unfounded negative, or they are simply in parallel with the main speakers.

Version line of passive level

So if you want to use separate amplifiers for the rear speakers, basically you can not - if you get smart. The first circuit shown in Figure 2 is completely passive, but requires that a suitable transformer. A suitable transformer means a line level, impedance of 10k units 1:1 - these are scarce, but are available after a search.

You may be able to get away with 600 units of ohms, but because the impedance needs, its performance is very common, with an extreme lack of bass (there are not enough inductance is 600 ohms transformer to operate at high impedance). Transformer is loaded to give back some of the low, but the preamp is likely to be very satisfied with the impedance. That said, I used this application to telecommunication transformers (600:600 ohm) and seem to work well.
Passive Line Level Hafler Matrix Decoder

The circuit is not a bad compromise, even if the impedances are too low to non-solid state preamp (preferably operational). Using a telephone adapter (600 Ohm), the loss is about 3 dB low frequency-3dB point around 100Hz. This varies depending on the quality of the transformer used, so experimentation is necessary. Although the 600 ohm telephone transformer are relatively easily available, some of them are quite common.

My tests were in a very good built by an Australian company called Transcap. I think I can say with some certainty that they will be reluctant to sell a single quantity. Another transformer manufacturer Midcom is very good in the U.S., but you will have the same problem with them. These manufacturers are prepared to deal with large orders from other companies, not people like you and I want ("Want a ...?") transformer. Therefore, you have to take everything you can get.

Since it is unlikely to be viable for most manufacturers, the alternative is to go active, using an ADC to perform the functions. This is described below.

The new circuit

The diagram in Figure 2 is a simple way to achieve the same (with some additional benefits) are online (ie, before the signal reaches the power amplifiers - in a bi-amp, this circuit is to be among the preamplifier and electronic crossover). The extras available are obvious ...

- The wiring is simplified (even if the power amplifier required)
- Now we have a center channel signal is available
- Provision for a mono signal to a sub-woofer is easy
The Schematic of an Enhanced Hafler Matrix Decode

While there have been published in the same circles over the years, this is a bit 'different areas. I wanted to avoid all the active electronics of the main left and right channels, as it eliminates any possibility of sound degradation due to the introduction of the threshold. Input impedance 50k does not cause any problems with the preamplifier (including types of valves), and the most important signal is simply a parallel circuit with the extras.

The volume is not included, because you already have a pre-amplifier. It would just become another component of the violin, and it would be little used, probably would have become a noisy time just sitting in a permanent position.

How it works

Opamp U1A is connected as an amplifier of the subtraction. If the same signal is used both as input, the output is zero. Consequently, it will remove all the background information from the stereo signal, and reproduce only the difference signal - exactly the same way as the original design Hafler.

U1B is a simple sum of the amplifier, and the food contains all the information as well as left and right channels. Ability to mind is that you could reduce the difference between the proceeds of this information, so the only material that is completely shared, both channels are reproduced. This would improve performance to the extent that an additional circuit is justified? I tend to doubt it, but you can pursue the matter.

Central control of the channel

Pot (VR1) is to set the center channel. This can be a TRIMPOT, or mounted on the back of a traditional dish (to help prevent the "fiddlers" from mucking your settings). I have seen circuits that do not have this, which basically feels like a bad idea. When two channels are added, the center channel is usually a level 3 dB compared to the left and right channels - if the signal is mono. Speech center channel (for example) is mono, so the standard is the same for each of the main speakers. Because the center channel amplifier and speakers are rarely as powerful as the left and right channels, it is very possible, the congestion amplifier, speakers, or both.

From the center channel is only supposed to fill "gaps" and provide a stable center of the image, which need not be so hard - especially because it is almost certainly lower than the main speakers sound quality and therefore reduce the overall sound quality. Level control lets you set the level just sufficient to provide a stable sound image, and no more. On my system, did not use a channel, and would have a negative impact on sound quality. If you have a good main speakers and a sound image stable and well defined, a center speaker can do more harm than good.

The capacitor (C1) is optional. Provides a nominal value of 8 kHz frequency roll-off (which apparently is quite normal for "real" surround sound processor). This helps minimize interference with the main stereo signal, but feel free to ignore, like most center channel speakers probably will not be able to play above that frequency anyway.

Subwoofer Out

The subwoofer output is simply taken directly from the mixing of the central canal, and I do low-pass filter, because I do not know any of the subgroups that do not have a filter in advance. Add another simply adds unnecessary complexity and will introduce the phase shift at the output of a phase compensation circuit (often included in the sub-woofer) may not be able to cope.


100 ohms at the exits to prevent the ability of the signal carried by the amplifier to oscillate. At that value, will not cause high frequency loss, unless you insist on 100 carries the signal of time (in my experience, these are rare).

It will also be noticed that there are two outputs for the rear speakers, just parallel. I included it because it is easier to thread if the user connects a stereo amplifier for rear speakers. Naturally, a mono amp do just fine as it is capable of performing the two rear speakers in parallel. It may not be possible if the speakers are 4 types of Ohm (it is becoming increasingly common in Hi-Fi, so it's not so stupid).

1 comment:

Kesava Raj said...

Left right i/p why using 100k...if we use 10k what happen...while increasing and decreasing resistor value what will happen...pls tell me sir