Active Surround Monitoring

We are facing a transformation in the technical paradigm of the audio field, whether it is for open dissemination, whether professional or aimed at the consumer. It is commonly called multi-channel ambient sound. The subject is huge and has many implications on production processes. The objective of this article is for the communications and audio engineer to learn about the great differences that exist between listening to stereo and listening to surround, as well as the different production opportunities. It is impossible to cover all aspects of surround production, but we hope that this small manual will help you see the "big picture" and understand the most important problems.

I-C-D-S; Left, Center, Right, Surround. This format relies on phase and amplitude ratios to "encode" 4 channels of audio on a standard 2-channel medium. Stereo productions in Dolby can exist in radio stereo, VHS and Beta tapes, analog audio tapes (including cassette), dual-channel digital audio tape, CD audio; in short, in any medium that enables two channels.

You need a Dolby set-top box (or its equivalent) to "extract" the 4 channels of audio for a standard 2-channel medium. Normally the four channels are referred to as Left, Center, Right and Surround.

*There is no specialized bass subwoofer and the surround channel may be limited, in terms of band, from 100Hz to 7kHz.

Dolby stereo is a 4-2-4 process often referred to as a 4-2-4 matrix.

Any audio event that is "out of phase" from left to right (I minus D) with equal amplitude, will decode or "lead" to the surround channel.

Any audio event that is "phased" relative to the left, right (I plus D) with equal amplitude, will decode or "lead" to the center channel.

• DTS. Digital Theater Systems.

This is one of the first discrete formats introduced in 1993. This format uses a single 20-bit wide digital bitstream and runs at 44.1 on CD/LD or 48kHz on DVD. It is based on actual band assignment and not on an analogous "matrix" type technology. DTS uses 5 discrete channels in addition to one LFE (Low Frequency Enhancement) channel. DTS uses a low amount of data compression to condense the audio into the digital stream.

• Dolby Digital AC-3 / Dolby EX
.

This is another discrete digital format, which originally carried the same 5.1 channels (the .1 being the LFE channel). Digital Dolby runs on multiple frequencies depending on the distribution format. This format uses a greater degree of data compression and has been called AC-3. AC-3 audio is the audio specification for open streaming and is not limited to 5.1 channels. In fact, AC-3 signals can be monophonic and even carry a Dolby stereo signal.

Dolby is lately introducing a new standard of a rear channel called EX, which adds a new surround center channel. The rear center is derived from discrete left rear channels and from the right in the same way as the center of the front is derived from the left and right channels in Dolby stereo.

• Sony - SDDS
.

This discrete multi-channel format is used exclusively for theatrical purposes and is not being distributed on disc for home or open broadcast uses. It was designed for large screen theaters since it uses 5 channels in front. It is a 7.1 format.

All discrete digital formats described above have full bandwidth for all 5 (or 7) major channels, except for the LFE channel, which is limited to 120Hz but is normally limited to 80Hz.

The transition from stereo to surround should not be as traumatic (or difficult) as the transition from mono to stereo.

We must recognize, however, that at the moment most high-definition audio for open streaming will be presented in Dolby stereo. This means that monitoring through a Dolby set-top box will continue to be part of the post-production process.

Only a few productions are made in 5.1 or 6 and those productions may even require a conversion process or downmix to Dolby stereo.

DOWNMIX: It is the process by which a 5.1 or 6 product is taken and the 6 channels are routed in a standard left and right mixture that is sometimes called "total" left and "total" right. This process could be automated but only after some experimentation, documentation and approval by all. That is why we want to say that there are some manufacturers that claim to provide products with the capacity to perform automatic downmixing . We respect, without a doubt, these statements but believe that it is wise and prudent to conduct some qualified assessments before committing to an automated procedure such as this.

The use of the central channel.

One of the first observations is the use of the center channel as the focus of the spoken word.

There may be other elements in the center, but generally the function of the central speaker is to "anchor" the dialogue.

Question: WHY?

Answer: Because it provides a solid foundation for dialogue (regardless of language) even when listening off-axis.

The left and right channels provide the SFX (Sound Effects) staging and musical details.

Rear channels can provide reverberation depth as well as dynamic front-to-back movement.

Question: Once you understand the finite resolution and technical nature of Dolby ProLogic, what are some of the problems one might encounter when performing a Dolby stereo production?

Answer: A lot of energy in an adjacent channel could take the dialogue out of the center channel. This decoding process is finite and sometimes the processor can mis-address the location of sounds.

This fact makes 5.1 productions easier to mix, but the mixer should not be tricked into making the wrong choices as to level and location.

This brings us to some very important topics:

There must be some minimum standards to which we can all adhere. Minimally one should:

1. Use the same speaker and the same symmetrical placement for left, center and right.

2. Set the levels for those speakers exactly the same: + / - 1/2 dB.

3. Recognize the frequency limitations of the front wall and draw the respective conclusions.

4. Use the same speaker or the same type of speaker for the rear.

5. Keep the same distance from the rear speakers as the front speakers.

6. Give them the same level.

7. Maintain a similar frequency balance between the rear and main speakers.

8. DO NOT ABUSE THE LFE CHANNEL.

Think. If someone ever thought it was simple to create a high-quality stereo environment, then how easy will it be to create a stereo environment with three times as many speakers? Not very easy. And remember, one of those speakers (LFE) is playing the most problematic audio source!

Acceptance of standards.

Below you will find some simple explanations of the 8 basic electro-acoustic rules for surround monitoring explained above.

Using the same type of speaker is priority number one, and for very good reasons.

The doorbell coupling is definitive and by having the same speaker combined with a symmetrical placement in front, it gives you the best chance to start in the right direction. More than 70% of audio production usually lies on the LCR (Left-Center-Right) wall.

Balancing the appropriate gain between all channels has all the common sense in the world.

If the front wall is made up of speakers that have a limited frequency response, say 80Hz, and you must make decisions regarding the frequency balance of a lower octave, you may have a problem. In this case, a subwoofer could be used to extend the performance of the front wall. This is a common form of bass tone management.

Generally those speakers that have a frequency reaction of up to 50Hz and that are used relatively close (<7 feet) with the resulting flat sound, can be used fully and can respond to 90% of production needs.

You could say that 5 speakers with the same specifications is the best solution. In some cases, the physical size will not allow this to occur. Depending on the amplitude requirements for the system, it is sometimes acceptable to downgrade speakers in a designation. Remember that the rear channels for AC-3, DTS and SDDS have no band limitation. In addition, depending on production requirements, monitors can be forced to perform extreme dynamics. It will always make a lot of sense to understand very clearly the performance of the rear monitors.

Generally, this is true. But remember: If you degrade the rear monitors due to size issues, you don't want to move the listening distance too far away from the LCR distance simply because you'll be forcing the rear speakers to work even harder. Whatever the decision, only a 25% increase in distance is allowed. As an example, if the measured distance LCR is 8 feet, the maximum distance at which the rear speakers of the production engineer must be should be 10.0 feet (8 x 1.25).

Ask him: Does this make sense? The answer is yes!

All GENELEC monitors use controls that allow the installer to adjust the LF and HF balances. These controls are usually adjusted depending on where the speaker is with respect to the nearest wall or a large surface. The goal is to have a similar balance when you are sending pink noise in sequence around all the monitors.

The LFE channel was designed to address the needs of additional low frequency (LF) effects and expansion. It was not designed to multiply the bass performance of the LCR or rear channels. The 5 main monitors should be able to function on their own.

However, in some cases they cannot. Therefore, some manufacturers offer some inputs that provide LF extension for the main channels, as well as for the playback of the LFE channel. In theory, this superior performance produces results, but it doesn't give the subwoofer a lot of work to do; especially if the total acoustic performance in the sound environment is expected to be more than 100dB! Remember that the LFE channel normally needs an additional 10dB above the main channels. This doesn't mean that the subwoofer fits 10dB more, but simply needs 10dB above the main speakers.

There are 2 reference points for the LFE channel: the production phase, where the engineer and producer must decide what signals they should send to the LFE bus, and the playback phase, where the subwoofer must be carefully considered and what it will actually be asked to play. This is a very delicate balance and, moreover, it means that the calibration of the subwoofer in relation to the main speakers is very important.

There is a tremendous opportunity, once it is established, to undertake new activities either for music, multi-channel sound effects creation, commercial post-production, free-to-air television and film. Multi-channel is no longer a novelty, it exists and is growing. The best preparation is to understand its fundamentals clearly.

Enjoyment!