Surround Sound Setup & Conversion FAQ

1. What's the difference between mono, stereo, and surround sound?

Mono refers to single-channel audio.

Stereo typically refers to audio with a left front channel and a right front channel.

Surround sound adds additional channels to stereo called surround channels. Sound from the surround channels is meant to come from the sides and/or back of the listener. The earliest form of surround sound for home use, quad audio, used a left front speaker, a left back speaker, a right front speaker, and a right back speaker.

2. What is a .1 channel?

Modern-day surround sound typically involves the use of a .1 channel, with 5.1 audio being a key example. What is this .1 channel? That depends on if you are talking about the audio itself or the speaker setup.

For the audio itself, the .1 channel is used for low-frequency effects (LFE). LFE sounds are deep, lower-pitched sounds such as thunder.

For speaker setups, a .1 indicates the use of a subwoofer, which is a special type of speaker used specifically for low-frequency sounds. A subwoofer is typically used to play not only the LFE channel but also low-frequency sounds from the other channels, sometimes called the main or satellite channels. When a subwoofer plays low-frequency sounds from the satellite channels, these low-frequency sounds are typically no longer played by the satellite speakers. Although it is sometimes possible to play low-frequency sounds on both the satellite speakers and a subwoofer, this is generally not recommended [1] [2].

3. How is a surround sound system typically put together?

Most people have either a 5.1 or a 7.1 surround sound system. The THX-recommended layouts for these types of systems are shown below (SBL = surround back left, SL = surround left, L = left, SUB = subwoofer, C = center, R = right, SR = surround right, and SBR = surround back right) [3]:

First up is a 5.1 layout:

Next is a 7.1 layout:

Still, these pictures are missing an important element of a surround sound system. That important element is the receiver, which is where the speakers are connected to. Audio is sent from the receiver in analog form to the satellite speakers over speaker cables [4] and to the subwoofer over an RCA cable [5]. Additionally, although the receiver and subwoofer need a power connection, the satellite speakers don't. I have modified the 5.1 picture to show these connections (green box = receiver, orange lines = speaker cables, purple line = RCA cable, black lines = power cables):

4. Why does my receiver give me the choice to set the satellite speakers to Small or Large? Which one should I choose?

Before answering these questions, it is necessary to go into a bit more detail about what a receiver does. Along with sending audio to the speakers, another important function of a receiver is bass (pronounced base) management. As was mentioned in question #2, a subwoofer can play not only the LFE channel but also low-frequency sounds from the satellite channels. In order for this to happen, the receiver has to direct low-frequency sounds from the satellite channels to the subwoofer. This action is called bass management.

The frequency at which the receiver begins to send low-frequency sounds from the satellite channels to the subwoofer is known as the crossover frequency. Some receivers let you have an individual crossover frequency for each channel.

With this background info out of the way, the original questions can now be answered. If a satellite speaker is set to Small, then the receiver only sends sounds at and above the crossover frequency to that speaker. However, if a satellite speaker is set to Large, the receiver sends all of the frequencies to the speaker. THX recommends getting satellite speakers that go down to at least 80 Hz, setting the crossover frequency to 80 Hz, and setting your satellite speakers to Small [6].

5. Is it possible to convert audio to a different number of channels?

Yes. Converting audio to a lower number of channels is called downmixing, while converting audio to a higher number of channels is called upmixing.

Getting surround sound from upmixing two or more channels is known matrix surround sound. By contrast, in discrete surround sound the surround sound channels are pre-generated or pre-recorded and do not come about from upmixing audio. It should be noted that it is possible to combine discrete surround sound with matrix surround, a common example of which is upmixing 5.1 audio to 6.1 or 7.1 audio.

There are two types of matrix surround sound. In basic matrix surround sound, the channels that are being upmixing have simply been recorded or generated as-is and haven't been optimized for matrix decoding. In encoding/decoding matrix surround sound, the audio will have gone through three steps:

  1. The audio is recorded or generated with more channels than will be transmitted.
  2. The audio is downmixed to the amount of channels that will be transmitted.
  3. The audio is upmixed to a larger amount of channels - this can be less than the amount of channels that were originally recorded or generated, the same amount, or more.

Because of these three steps, encoding/decoding matrix surround sound is sometimes described in the x-y-z format, where x represents the number of channels in step 1, y represents the number of channels in step 2, and z represents the number of channels in step 3.

6. How has matrix surround sound been used?

One of the most important companies in matrix surround sound has been Dolby Laboratories, founded by Ray Dolby in May 1965 [7 1965]. A large reason for this importance is that when surround sound is encoded for matrixed decoding, it is often done so with Dolby decoding in mind. THX has also played a role in matrix surround sound, albeit a smaller one. Below is a timeline of Dolby's and THX's matrix surround sound technologies:

7. Why are the SBL and the SBR speakers in the 7.1 picture red?

To answer this question, it is necessary to take a closer look at THX's speaker layout recommendations. THX actually makes two different recommendations for a 7.1 speaker layout. If a 7.1 surround sound system is playing more than 5 channels of audio only with THX Surround EX or THX ASA, THX recommends having the SBL and SBR speakers together. On the other hand, if a 7.1 surround sound system is also playing 7-channel discrete surround sound, THX recommends having the SBL and SBR speakers apart [3].

With this background information, we can now answer the original question. THX makes the SBL and the SBR speakers red in the 7.1 picture to remind those with a THX receiver to adjust their THX ASA settings for having the SBL and SBR speakers apart rather than together [3].

8. What is personal surround sound and virtual surround sound?

In personal surround sound (a.k.a. virtual surround sound), surround sound audio is downmixed to stereo in a manner that attempts to preserve the directionality of surround sound using only a standard pair of headphones or two external speakers. The techniques used for this approach typically differ depending on if the personal surround sound is listened to with headphones or with external speakers.

Dolby has two personal surround sound technologies: Dolby Headphone and Dolby Virtual Speaker [44] [45]. Dolby Headphone was announced on Dec. 8, 1998 and is meant for headphones [46]. Dolby Virtual Speaker was announced on Oct. 29, 2002 [47] and is meant for two external speakers [44] [45].



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