BASIC CALIBRATION

All AV receivers and processors perform three essential functions. First, they adjust the timing of the signals from each channel. Second, they adjust the level of each channel. And, third, they manage how the bass is handled. Consider a basic 5.1 channel sound system. It may be delivered in the form of Dolby Digital or DTS, or one of the lossless formats such as Dolby TrueHD or DTS-HD Master Audio. Of course, the receiver can decode that, and the result will be five full-range channels and a Low Frequency Effects channel intended for the subwoofer. The LFE channel only handles frequencies below 120Hz and is where the sound engineer will put dinosaur footsteps and earthquake rumbles…

Each of those channels assumes a certain layout of speakers. It expects that the five main channels – front left, centre and right, surround left and right – are capable of handling a full range of frequencies, down into the very deep bass. It expects that all speakers are the same distance from the listener and will each produce the same sound level for the same input signal. You can build a home theatre like that, but few do because it requires a huge room and large, identical loudspeakers for the five main channels. 

Instead, most home systems have better speakers for the front left and right, a lesser speaker for the centre channel, and lesser still speakers for the surround channels. Sometimes the front left and right handle the deep bass, but rarely can the centre or surrounds. Even in a well set-up system, the surround speakers are almost always closer to the listeners than the other three. Sometimes real-world room geometry can make it hard to even form the proper isosceles triangle with the front speakers at the base and the listener at the apex.

That geometry and those levels are important. The way our binaural hearing determines the direction of sound is based on the level of the sound from different directions, and the timing. If you hear the same sound coming from two different sources, the one that hits your ears first will be the direction that you sense. This is called the Haas Effect or Precedence Effect. The first sound to arrive takes precedence over the same sound coming from elsewhere – even if the second sound is up to 10dB louder. 

The main setup menu of a Denon home theatre receiver. The organisation will be a little different on other brands, but most have comparable facilities.

Since surround sound is about making sounds appear convincingly from locations all around the room, not just from individual speakers, the timing has to be right to make it work. If the surround speakers are closer than the front speakers, then those channels will need to be delayed a little so that they are timed to match the front speakers. The sound levels should be balanced as well. Surround speakers are often of lower sensitivity than the front speakers, and so quieter – but they are also usually closer, and so louder. Finally, the bass in the channels going to the smaller speakers is best redirected to a subwoofer, or in its absence, the larger front speakers.

What about that sense of direction, you ask? If you send the bass from the surround speaker to the front speaker or a subwoofer, won't that wreck the surround image? Bass frequencies have long wavelengths, so the ear can't judge the timing well enough to develop a sense of direction. Instead, our brains correlate those bass frequencies with their upper harmonics, delivered by the other speakers, and ties them together. So the bass will seem to be coming from the same place as those higher frequencies. We have pretty amazing signal processors in our heads!

All this processing is vital for effective home theatre performance. All AV receivers and processors worthy of the name can handle all this, and most automate it. This involves placing a microphone – included with the unit – in the position of the listener's ears. The receiver then runs a series of test signals and determines three things: the delay needed for each loudspeaker, the level adjustment needed for each speaker, and a crossover frequency for redirecting bass from each loudspeaker to a subwoofer (or from centre and surrounds).

The results of an automatic calibration by the receiver. You can tweak these manually, but you should never need to.

I've reviewed dozens of home theatre receivers and surround processors, and all of them do those first two things very well. They produce accurate results for both distance and level for each speaker. If you have a very old receiver, or one that's very strange, you may have to make those adjustments manually. For levels, you will need a sound pressure level meter. Since we're primarily concerned with relative levels, an app on your phone will do the trick nicely.

For the delay, you'll need to measure the distance between each speaker and the listening position. Sound takes around 3 milliseconds to travel 1 metre. If the front speakers are 4m from the listening position and the surrounds are 2m, then the sound to the rear speakers should be delayed by 6 milliseconds – 2m difference times 3ms per metre.

In many home theatre receivers you can adjust the frequency below which bass will be redirected from speakers.

The automatic systems in home theatre receivers need to set the size of your speakers, and the crossover. In home theatre, 'Large' means full range and 'Small' means that some of the bass should be sent elsewhere. Most systems allow you to set a crossover frequency for 'Small' speakers. Many of them allow you to set different crossover frequencies for different sets of speakers. So you might want your front floorstanders to handle everything down to 40Hz, and send the lower content to a subwoofer, and/or relieve your small surround speakers of any responsibility for bass below 80Hz.

Home theatre receivers have to make a judgement as to the size of the speakers based on their measurements. They've gradually been getting better, but many still make mistakes. After running the automatic system you should go into the manual 'speaker settings' and check what have been made, and correct them. It is not critical to determine exact crossover frequencies. In general, it's best to consult the specifications for your loudspeakers and set the crossovers to a little above what the specifications say they can deliver.

Do keep the crossover point to no more than 120Hz for all speakers, if possible. As higher bass signals get redirected, they're more likely to mess up the correct perception of distance. If you're using small satellite speakers for which a higher crossover is recommended, experiment a bit. Try the recommended crossover and play material with bass content. If you find that you can identify any sounds that seem to be coming from the direction of the subwoofer, try lowering the frequency until the subwoofer's location becomes sonically invisible.

You can manually adjust the sizes of the speakers, and may need to. Large means no bass redirection, Small means that bass will be redirected. You can’t set the front speakers to Small unless you have a subwoofer.

EQ

Most automatic setup systems in home theatre receivers also equalise the room and your loudspeakers. In the culture of purist hi-fi, equalising the frequency balance to correct speaker inaccuracies is frowned upon, whereas doing so to correct room effects may be considered more acceptable. In the real world, no system can easily tell them apart – is that dip at 60Hz because of room effects or due to your speaker's response falling off naturally? Your choice is to accept the EQ the system applies, or to switch it off.

Some high-end calibration systems such as Dirac let you set your own 'target' for EQ – in other words, the sound balance that you personally want. But even lesser ones often let you set the desired EQ to 'flat' – the hi-fi ideal – or 'natural' or something similar, or 'front'. 'Natural' tends to soften the high frequencies somewhat. The idea is that in a real cinema, with much larger distances for the sound to travel, the higher frequencies are naturally attenuated as they pass through the air. 'Front' leaves the front stereo pair alone and EQs the other speakers to match them.

Audyssey is one of the automatic calibration systems used in some brands of home theatre receiver.

Some automatic calibration systems go further and look for other non-linearities such as room-induced phase distortions, and group delays in different frequency bands which they can adjust. I'd love to suggest hard-and-fast rules on whether to use any of these, but it's really something that you should experiment with, and see what you like the most. If you have high quality loudspeakers, you may be able to get away without EQ or other adjustments – but your speakers may benefit from this treatment.

One good tip is that most home theatre receivers have a sound mode called 'Direct' or 'Pure Direct'. Check with your manual, but in most cases this switches off the EQ. 'Pure Direct' will often switch off more processing too, and/or the front panel display. Often 'Direct' also stops bass redirection.

On my own reference Denon AVR-X3500H receiver, I use EQ and bass redirection when watching movies or Netflix or Prime Video. But when I want to listen to stereo music through it – whether streaming via the network or spinning a disc in an external player – I tap the 'Direct' button on the remote and the sound is delivered by two KEF R300 speakers alone, no subwoofer involved, and no EQ or other processing. For really serious listening I go to my dedicated stereo system, but that's another matter…

SOUND IMPROVEMENT DSP

Since home theatre receivers all have digital signal processing power built into them, most also offer certain sound treatments which are claimed to improve the sound over that delivered by the source. All such things should be viewed with deep suspicion…

Some processors feature so-called smart dynamic compression systems – in Audyssey systems it goes under the name of 'Dynamic Volume'. This is designed to keep the full range of the sound – from soft to loud – audible without overloading your system. This should be left off by default. Perhaps if you're doing some late-night listening and don't want to disturb others, then switch it on.

Something which should never be used is 'Dynamic EQ', or equivalent. This adjusts the tonal balance to take into account the output level. The idea is to correct for the fact that our ears don't hear high and low frequencies as well as the midrange at low volume levels, rather like the 'Loudness' button on classic hi-fi amplifiers. This process is truly horrible, because our minds expect treble and bass to be subjectively lower at reduced volume levels. If that expectation is not met, the sound seems unnatural. Leave that switched off!

Many systems have a processor intended to restore content lost in lossy compression systems. They either do nothing or add stuff that shouldn’t be there. This image shows the correct setting for these.

Most home theatre receivers connect to your home network and stream digital audio from local network resources as well as certain streaming services. Many include a process called 'Restorer' or similar, claimed to add back into the sound elements removed due to lossy MP3 or similar compression. However, they simply cannot do that – at best they tweak the frequency balance, and at worst they make up stuff which they think that might have been lost due to the compression. Leave it off.

SURROUND PROCESSING

So now I seem like a luddite, rejecting all enhancement innovation. Not quite. Give careful consideration to switching on Dolby Surround. This is the latest Dolby processor which can take whatever input signal – two channels to 5.1 – and extract channel information for all your surround speakers, including any ceiling ones.

I'd be reluctant to use this for stereo music, but movies with surround sound delivered in two channels – typically from broadcast TV – frequently retain a lot of surround content embedded into the audiostream. These can sometimes deliver impressive results. I also routinely leave Dolby Surround on for 5.1 channel shows from DVD, Blu-ray, UltraHD Blu-ray and streaming services. Sometimes the height extracted from the other channels can seem indistinguishable from Atmos.

LIP SYNC

In most cases, just set this to Auto. But manual adjustment is usually possible, and sometimes necessary.

Let's finish off with one slightly different and seemingly simpler bit of sound processing – lip sync adjustment. This is only important for A/V content. Modern TVs and projectors do much time-consuming video processing. The 'Standard', 'Natural' and 'Cinema' display settings take a lot of number crunching to do their thing, so the display of the image on the screen is always delayed. For those settings (in 'Game' mode the delay is reduced significantly), the picture is usually delayed by somewhere between 100 and 150 milliseconds. That puts it out of sync with the audio. It's disconcerting to watch a show where the mouth and voice aren't quite in sync. 

Home theatre receivers include a simple delay which brings the audio back into sync with the picture. For quite a few years, HDMI supported automatic lip sync adjustment. When first switched on, the receiver queries the TV over HDMI, which responds by telling it how much delay is required. Most of the time this does the job perfectly well, but I've seen some TVs which don't support this, or have an incorrect value for the delay. Sometimes if you change picture mode on the TV you can change the picture delay, but this isn't communicated to the receiver. So there may be a visible lip sync problem. In those cases you can switch off automatic lip sync and manually adjust the delay.

This can be hard to get right, especially if you're just trying to do it by matching dialogue to moving lips. That same evolution that helped us hear where the dangerous creature was located also helps us match sound to lips. Remember, if someone is shouting to you from twenty metres away, the sound of their voice is delayed by 60 milliseconds. Yet your brain adjusts the timing so that it looks like the lips and words are in sync.

The top line lets you set the EQ target. The Reference setting is one preferred by the calibration system, but it can be changed to Flat or L/R Bypass, or just plain switched off.

THE FINALE

So there you have the basics of what your home theatre receiver can do for the sound of your system. Some of those things are essential, while many are to be avoided. Consider all this as the basis for your own experimentation, because the object of home entertainment is not technical purity, but the entertainment of you and yours in your home!

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