Setting up a 6.1/7.1 multichannel system

For a little under a year, A/V receivers capable of amplifying 6 or 7 channels have been available on the market, going beyond the traditional configuration of 5 channels plus subwoofer. On paper, this new configuration aims to be more attractive and spectacular, since adding a new independent channel at the rear is assumed to provide a greater sense of surround sound, coming ever closer to emulating the sound of a movie theater in our living room.

Even so, adding this channel (which ultimately means one more loudspeaker) is no easy task, and is sometimes impossible in certain spaces. Below we explain these new extended formats, the requirements that must be met before undertaking an installation of this caliber, and a series of practical tips that we hope will help those who do not yet have a Home Cinema system or who aspire to improve their current one.

Once again it was George Lucas who helped Dolby Laboratories popularize a new format, with the theatrical screening of "Star Wars, Episode 1: The Phantom Menace" in Dolby Digital Surround EX. This 6.1-channel professional digital multichannel audio format added a third surround channel. For its DVD-Video release, the "home" version was developed: Dolby Digital EX. The main advantage of this format is that it is fully compatible with 5.1-channel Dolby Digital decoding systems. This means not only that all 6.1 sound can be played back in 5.1, but also that no sound information whatsoever is lost. The secret is that the information for the additional channel is matrix-encoded. In fact, instead of multiplexing six independent channels (plus the low-frequency effects channel), the new channel is embedded in the two surround channels. If a 5.1-channel decoder is used, the information of the supposed sixth channel appears equally in the two side channels, whereas if an extended system is used, this information is only reproduced by the corresponding channel. If Dolby Digital developed its format, it was to be expected that its natural competitor would do the same. Thus, DTS responded with DTS-ES, very similar in concept but with two different versions, also compatible with DTS 5.1 decoding systems: Matrix and Discrete. The difference lies in how they encode the sixth-channel signal. The Matrix option is similar to the one adopted by Dolby (it embeds the information in the rear channels), while Discrete keeps the sixth-channel information independent in discrete encoding.



WHY A NEW CHANNEL?



According to Dolby, for a better three-dimensional sense of space at the rear. Until then, there were two independent audio channels for the surround section. As conventional movie theaters grew larger and Home Cinema became popular, a lack of freedom of movement at the rear became apparent. By adding a third channel, moving sound effects achieve a more spectacular and extended trajectory.

There is another answer: the need for new developments that encourage industrial and commercial progress. The market constantly needs new products and solutions, in both the professional and home environments. The addition of these standards is not only adding spectacle to movie soundtracks, but is also leading many users who until now enjoyed 5.1 systems to consider acquiring a superior system. In professional environments, it encourages the purchase of a new audio decoder and the addition of a new power amplifier dedicated to the third channel.



TECHNICAL REQUIREMENTS



Fortunately for enthusiasts, the move from Dolby Digital or DTS to Dolby Digital EX or DTS-ES is fully compatible with any DVD-Video player capable of reading Dolby Digital or DTS (a high percentage, if not all of them). This means that the only audiovisual component specifically required for compatibility with these new formats is the A/V receiver.

The connection between the DVD-Video player and the 6.1 or 7.1-channel A/V receiver is made as always: a digital audio connection via an EIAJ-TosLink or S/PDIF link. The DVD-Video player reads a binary data stream in 5.1 just as it does one in 6.1. In the same way that it reads them, it transports them digitally to the receiver. The receiver must be equipped with the corresponding decoders, which in the case of conventional 5.1 will be Dolby Digital or DTS, and in the case of 6.1/7.1, Dolby Digital EX and/or DTS-ES. There are no A/V receivers with extended decoders and only five amplifier channels since, as we have seen previously, Dolby Digital EX and DTS-ES streams are fully compatible with 5.1 decoders.

Without yet addressing the question of 6 or 7 amplifier channels (that is, adding 1 or 2 loudspeakers), we now turn our attention to the loudspeakers. In the case of a new installation, the only requirement is to get hold of a set of electrically and tonally matched loudspeakers; for example, a combination from any particular series from one manufacturer. But what happens when we already have a 5.1 loudspeaker set? For the movement of sounds to be smooth and fluid, it is essential that all the loudspeakers we use have the same tonal balance and performance. An A/V receiver is, among other things, a multichannel amplifier (that is, a set of mono amplifiers) whose levels we control using a single knob (normally the volume control). By turning it, we set the same power value for the whole set of built-in amplifiers. But the power of an amplifier does not necessarily correspond to the sound pressure of a loudspeaker. The synergy between amplifier and loudspeaker (involving variable values such as impedance and sensitivity) will mean that one loudspeaker achieves a higher sound pressure than another using the same amplifier and volume level.

Even if we find two loudspeakers from different manufacturers (or from the same manufacturer but different series) with identical sensitivity and impedance values, there is no guarantee that, given the same current delivery and the same sound, they will sound perceptibly the same. There will be differences, and noticeable ones.

All of the above serves to justify the pressing need for anyone upgrading their 5.1 installation to 6.1/7.1 to pay special attention to the choice of loudspeakers. The most correct approach is undoubtedly to choose the same model and series for the surround channels, so that the overall tonal balance at the rear is homogeneous and correct. Sometimes it will be impossible to find a loudspeaker on the market identical to the ones we already have, so we either buy new loudspeakers for the whole system or look for a loudspeaker as similar as possible to the existing ones. An arduous and subjective task.

Loudspeaker placement in 6.1/7.1. In blue, placement of the surround loudspeakers in a conventional 5.1 installation. In red, ideal placement of the loudspeakers for a 7.1 configuration. If only one loudspeaker is used for the sixth channel, it is recommended to place it directly behind the ideal viewing position ("E").

PHYSICAL REQUIREMENTS



Following the publication of the new multichannel audio standards, Dolby gave a more precise name, "side channels", to what we had until then called "surround channels" or "rear channels". This new definition is somewhat more accurate, inasmuch as Dolby recommends placing them at the sides of the viewer, at exactly 90º. But we still favor 110º for the surround channels in 5.1-channel installations, as this allows better sound diffusion at the rear. We agree with Dolby on the placement of the side channels with respect to the vertical axis: noticeably above the viewer's head. If we use a 6.1/7.1 system, the first thing to do is to reposition the surround channels at the sides, at 90º to the viewer. The sixth channel should be placed directly behind the viewer (180º) and at a distance similar to that between the viewer and the side channels. If two loudspeakers are used for the sixth channel, we will maintain the same height, reposition the former surround channels at 90º, and the two new loudspeakers will be side by side behind us at 180º, evenly spaced at a minimum distance of 1.22 meters (4 feet).

In both cases it is obvious that anyone whose sofa is up against the wall will hardly find a 6.1/7.1-channel system attractive, given the difficulty of placing the sixth channel more than a meter behind them. Remember that the sixth-channel sound is not lost in a 5.1 configuration, but is reproduced equally in the two side channels (if they are well positioned, the acoustic sensation will be that the sound source is directly behind us).



SOFTWARE



Finally, a short appendix on the various DVD-Video titles that already feature 6.1-channel soundtracks. The vast majority usually include the corresponding logo on the back of the case. Once the disc is placed in the player's tray, the system detects the presence of the corresponding soundtrack and selects the appropriate decoder. However, it was not until 2001-2002 that the detection "flag" was incorporated. The "flag" is the code needed for this automatic switching. With movies released before 2001 or 2002, even if we know they are in Dolby Digital EX, our receiver may not switch to the corresponding decoder, so we will have to do it ourselves.

In much the same way, and for no apparent reason, some studios are not aware that the soundtrack of the movie they are releasing is in Dolby Digital EX or DTS-ES, so they do not put the corresponding logo on the cover. Today, many film productions use Dolby Digital Surround EX as the standard for their sound, and in 99% of cases this soundtrack is retained in the DVD-Video version.

Differences between 6.1 and 7.1

There is no such thing as 7.1 sound, understood as the amplification of 7 independent audio channels plus subwoofer. Dolby Digital EX and the two versions of DTS-ES offer 6 channels. Even so, it is easy to see that there are 6.1-channel receivers and 7.1-channel receivers. This distinction defines how many independent amplifier channels each A/V receiver has: a 6.1-channel receiver will have six amplifiers, while a 7.1-channel receiver will have one more discrete amplifier. But both will only be able to reproduce the 6 channels of information of the extended multichannel audio formats. Why one more amplifier channel? The difference lies in using three or four loudspeakers for the surround section. A 6.1 installation adds a single loudspeaker, whereas a 7.1 installation allows two loudspeakers to be used for the "extended" channel without sacrificing impedance or using separate power amplifiers, with the ultimate aim of achieving the best synergy.