Multichannel soundtracks on a stereo system: you may be losing more than you think


Mis à jour le 26 February 2019.


As you?ve probably noticed, multichannel sound is no longer exclusively limited to DVDs, Blu-ray discs and 4K Blu-ray discs. HD DTV, Netflix, Amazon Prime Video, iTunes, and many US networks (AMC, Starz, etc.) now broadcast and stream programs in formats such as Dolby Digital Plus (DD+/E-AC3), stereo surround sound, 5.1 surround sound, and even Dolby Atmos for some content. If your installation includes an AV receiver, 5 speakers and a subwoofer, (and perhaps Dolby Atmos ceiling mounted speakers), you will be able to enjoy the entirety of the original mix, and you will be fully immersed in the atmosphere designed by the audio engineer. On the other hand, without a center speaker and subwoofer, or with only two speakers to deliver the entirety of a Dolby multichannel soundtrack, you might be missing out on more than you think.

Downmixing, according to Dolby

When one or several speakers are absent, the multichannel decoder integrated into a TV, set-top box, or AV receiver has to downmix the signal from 5.1 channels to match your system?s format: 3.1, 4.0, 2.0, etc. Meanwhile, not all the information in 5.1 soundtracks goes through the same mixing process? some of it is simply not mixed at all.

Since the early days of Dolby Digital in the 90s, Dolby-certified multichannel audio soundtracks have been designed to ensure compatibility with systems composed of less than 5 speakers. All Dolby 5.1 soundtracks (Digital, Digital Plus, True HD) contain additional metadata which may be used by the decoder to distribute sounds intended for absent channels between the available speakers. All TVs, 4K Blu-ray players and AV receivers use this metadata to downmix 5.1 soundtracks.

The Sonus Faber Principia C center speaker.

Without a center speaker

Without a center speaker, the center channel is mixed in stereo and distributed between the front left and front right speakers. The sound level is reduced to half (-3dB) in order to deliver the original signal at the right volume with two speakers. This is where things get tricky, and if the audio engineer decided to further reduce the sound level during the downmixing process (maximum reduction of -6dB), then the amplifier will have to comply with this obligation.

This is why dialogues in certain movies are a lot quieter than the rest of the soundtrack, as they are drowned out by other information in the left and right channels.

Without surround speakers

Following the same logic, the metadata of a 5.1 soundtrack will be mixed in stereo and distributed between the front left and right speakers. Sound attenuation may vary from one movie to another as well. It is even possible for the surround channels to be completely excluded from the restitution.

The Jamo D6000 SUR surround speaker.

Without a subwoofer

No subwoofer, no LFE channel (the famous .1 channel). Dolby does not offer the possibility of mixing the low frequency channel into the stereo signal for delivery through the front speakers. This is to be expected, as the LFE channel delivers frequencies between 20 Hz and 120 Hz, often at high volume. Many front speakers and receivers would struggle to deliver this frequency range with any clout (the extensive cone excursion could damage the driver, and the amplifier may fall short in terms of power). Nevertheless, some AV receiver manufacturers offer the possibility to downmix the LFE channel and direct it toward the front speakers, although this option puts a significant amount of strain on the receiver (which could negatively impact the restitution).

In a nutshell, without a subwoofer, no deep bass and no physical impact.

The Focal Cub Evo subwoofer.

Without an AV receiver

If you are using a stereo amplifier fitted with digital inputs, your optical player or TV will downmix the tracks into a stereo signal. The downmix carried out by a stereo amplifier will always be inferior to the signal restitution ensured by an AV receiver. A receiver is equipped with a DSP and can oversample 16-bit/48kHz signals to 24-bits/96kHz in order to ensure high-resolution sound.

The absence of an AV receiver also means no dynamic range compression, which is essential to enjoy all the details of a 5.1 and 7.1 soundtrack at low volume. Often referred to as ?night mode? or ?loudness?, dynamic range compression (DRC) is only offered by AV receivers and some high-end Blu-ray players.

The Yamaha MusicCast RX-A870 AV receiver.

Dynamic range compression: working principle

Metadata encoded in Dolby tracks indicates how the sound level of each track should be adjusted so the amplifier may scale down the dynamic range while preserving the balance of the original mix (as much as possible). There are two types of DRC modes: DRC line mode and DRC RF mode. Not all AV receivers offer both options, in which case only the DRC line mode is available. Moreover, DRC RF mode is slowly disappearing as it excessively compresses the dynamic range (as it does with DTV, for example), which results in a particularly dull soundstage.

The DRC is calibrated by the audio engineer with a 5.1 and 7.1 installation in mind. If one or several speakers are absent(the center speaker, for example), the compression carried out by the receiver will be minimal. The best option to avoid having to constantly adjust the sound level is to have the appropriate number of speakers.

5.1 and 7.1 surround sound from a stereo signal: does it really work?

You?ve probably noticed that DTV channels feature 2.0 Dolby Digital Plus surround sound. Nevertheless, this is not a classic stereo (2.0) soundtrack. TV channels actually downmix the original 5.1 tracks. This downmix is carried out using the metadata contained in the original soundtrack. A 2.0 DD+ soundtrack is comprised of sounds recorded in phase inversion and a marker so that the AV receiver may upmix the soundtrack for surround sound systems. The user shouldn’t have to force-activate a surround sound mode (Dolby Pro Logic, DTS:Neo, etc.), as the Dolby Surround mode should be automatically activated. Consequently, a 2.0 signal upmixed for a 5.1 or 7.1 installation is not only relevant, it also works like a charm, especially with DTV channels.

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