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Sound Card Audio Formats

 

Overview:
Well, now that the sound card in installed, we need to know how to operate it. For the most part, using it will be as easy as it was when you were using the on-board sound card. But... Sometimes, you will run into strange problems. To help you through the software related problems that may arise, I will try to cover most of the audio formats and the speaker configurations. Some of this is going to be relatively technical. I will try to provide information that's detailed enough to help get even the most technically challenged through it with no more than a few cuts and scrapes. :)

The Basic Sound Configurations

Two Channel Stereo Audio:
Many people have only 2 relatively small speakers on their computer. That's perfectly fine for most computer use and web surfing. Others want higher quality audio from their systems and this generally means that a subwoofer is added to the two speaker setup. For either of these setups, use is pretty straight-forward. For the 2 channel system without the sub, the front channel output is sent directly to the speakers (the front speakers plug directly into the sound card). For the system that has the subwoofer, the front channel output of the sound card is fed to the subwoofer. The subwoofer's internal crossover filters the bass from the signal that's sent to it from the sound card and it sends the remaining signal to the small left and right speakers. The following image shows a basic two channel system with no subwoofer.

Notes:

  • Virtually all computer speaker systems include their own amplifiers. The output of a sound card is usually very weak (can produce very little electrical current -- and therefore can produce very little power). This means that sound cards can not drive speakers directly. Some sound cards can drive headphones from the front outputs (or some other specific output) but the rest of the outputs are strictly pre-amp level outputs (they can drive nothing more than the high impedance input circuit of an amplifier).
  • A sound card that can drive headphones directly will only be able to drive headphones with relatively high impedance. Most newer headphones have a relatively high impedance (32 ohms is very common). If you have an older set of headphones, their impedance may be as low as 8 ohms. No sound card will be able to drive an impedance that low without significant distortion. If your headphones don't sound good when plugged into your sound card and you have them plugged into an output jack designed to drive headphones, your headphones may have an impedance that's too low.

    This image shows the two channel system with a subwoofer (under the desk).

    Multi-Channel Audio:
    Most sound cards (even integrated sound cards) have more than one pair of stereo output channels. This means that, with a multi-channel signal, the output signals of the sound card reproduce more than 2 discrete audio programs (more than the normal left/right signals). In the simple system in the previous paragraph, there were 'three' speakers but there were only 'two' discrete signals driving them (the signal was split to the three speakers beyond the output of the sound card). For the simplest multi-channel system (beyond two channel systems), you have 3 channels of full range audio plus one .1 channel (a bandwidth limited channel). Such a speaker setup would look like the following image. You should know that this type of system can be derived from a simple 2 channel signal but will produce the best overall quality when it's produced from digitally encoded data on the DVD.

    The next level of complexity is the 4.1 channel setup. This, like the 3.1 system, can be produced from a 2 channel signal. The surround channel is a mono signal. If the signal is derived from a signal source that has the information to produce two independent surround channels, the signals will be mixed down to the single channel.

    This is a true 5.1 channel system. Most DVDs can be encoded with the data to drive each of the 6 speakers with 6 discrete channels of audio.

    Note: a 'discrete' channel is a channel that can reproduce a signal that is in no way dependent on the content of any other channel.

    Digital Surround:
    Most all DVDs sold in the US have digital surround encoded into the audio data stream. For the most common system (5.1 channels) there are 6 discrete channels. Five of them are capable of producing full range sound. The sixth (0.1) channel is only designed to produce low frequency effects. I think I should make it clear that the LFE channel is NOT the subwoofer channel. The 5 full range channels contain the bass content for the soundtrack (for the bass in music and such). The LFE is for effects.

    Home Audio vs Encoded DVD Signals:
    Many high-end home audio systems employ a sub/satellite speaker system. In this type of system only higher frequencies are sent to the satellites (typically 2-way speakers with woofers that are 6.5" or smaller). All of the low bass is sent to a subwoofer. On a DVD, the digitally encoded 5.1 channel program delivers a full range signal to all of the 5 speakers (left/right front, left/right surround and center). Generally, the only signal that's sent to the subwoofer is the LFE (Low Frequency Effects) signal. The LFE signal is what's used to make the super low frequency rumble and vibration.

    Additions to the LFE Signal:
    Since few satellite speakers are capable of producing really low bass and since the LFE signal doesn't get much use (differs from one movie to another), the subwoofer can do double duty. In one of the control panels in the audio software, you can tell the decoder to redirect the low frequencies from the main soundtrack to the subwoofer.

    Signal Levels

    If you only use one media player (Windows Media Player, for example), you have likely never had the following issues but you should read this because you will most likely be installing more/different media players in the future. The problem arrises when the other media players access different level controls to set the volume. Some adjust the 'wave' level. Others adjust the 'master' volume.

    Multiple Volume Controls:
    We touched on the volume controls on the 'desktop' page near the beginning of this tutorial. This is basically the same but you will see that there are several level controls. Some of which are not all that visible (unless you know where to look). The following is the typical volume control/mixer. It can be accessed by double-clicking the volume control icon (the little speaker) in the systray. If the icon isn't in the systray, go to START >> ALL PROGRAMS >> ACCESSORIES >> ENTERTAINMENT and select the volume control.

    This image shows the volume control icon. It's the third from the left in the following image. Also notice the icon on the far left. We'll discuss it shortly.

    There are two things you need to take note of in this next image. The first is the 'Mixer Device' drop-down menu. When you click the 'down-arrow', it expands. If you have more than one set of sound card drivers installed, you need to make sure that the proper one is selected. Here, I have a choice of the Turtle-Beach card (that I recently installed) or the on-board card (the Via AC97 audio device). If you have the wrong device selected, the mixer's controls may not work properly or there could be some missing/unusable options/features shown. The next thing you need to notice is the check boxes in the lower pane. This is where you select the controls that you want access to on the mixer. There are generally a few that you don't use and therefore don't need on the mixer. You can likely leave it as is.

    Below (just for this example), I selected most (maybe all) of the possible choices. As you can see, I had to shrink it considerably to get it to fit in this 800 pixel wide column of text. On the screen, it spanned just about across the desktop.

    Below is a second picture of the mixer properties dialog box. As you can see, I've expanded the 'Other' menu. There is only one choice here (there could be other preset choices) but you would select the choice shown (for the 'all channels' display).

    This is a screen cap of the 'all controls' mixer. Notice that I've clicked the 'other' radio button. These controls can be accessed in several ways on this computer (with the mixer shown, in the Catalina control panel or in the AC3 filter control panel). These controls determine the output level of the individual output channels on the sound card.

    As I mentioned, you can access the controls through the Catalina control panel. Remember that icon I showed you earlier (the one that's 'supposed' to look like the icon on the bottom-right of the following dialog box)? Clicking it will launch the following control panel. This control panel and the mixer I showed you immediately above are different interfaces for the same controls. If you have them both on the screen at the same time, you can move one set of controls and the slider on the other interface will move in unison.

    I'm showing you all of these level controls because it will be necessary to be able to find them if you have problems with no audio or insufficient output from one or more outputs on the sound card. You shouldn't forget about one more level control. It will be the resident volume control on the media player you're using. Some are linked directly to one of the sliders shown here and other are independent.

    Notes:

    • The icon that launched the control panel for my sound card's controls will not look like yours unless you have the same sound card. When installing the software for your sound card, take note of the names AND icon that will be displayed on the installation dialog boxes.
    • If you have no sound in any one application, don't forget to check to see if its 'mute' button has been pressed.
    • If you don't have sound on one particular input line, be sure that its slider is set to at least 1/2 volume and that it's not muted (checkbox below slider on mixer).

    Audio Content in Various Channels

    When using only the front channel outputs on the sound card (that's what you would use for a two channel stereo speaker setup -- with or without a subwoofer), the setup is generally done by default. When you move to a system with a '.1' channel, the setup becomes a little more difficult and somewhat unpredictable. If you have a 5.1 channel speaker system (very common), and you're playing a CD or DVD that was produced with only two channels of audio (left and right), you need to know about the check boxes in the next two images. In a true 5.1 system, you have 6 discrete channels. If you have only 2 channels of audio, you will get audio out of only 2 channels (left/right front). Since the front speakers are likely satellites (small speakers), they will not produce any appreciable bass. To force audio out of the other speakers (including the subwoofer), there is generally some sort of feature to distribute the audio. In the following dialog boxes, the 'Stereo Expand' and the 'Magic 5.1 enable' cause the audio to be played through all of the speakers. The name of that feature on your system may be different. Look in the owner's manual if you can not find it. The owner's manual will likely be on the installation CD in PDF format.

    Note, to access the box above, click 'advanced' under the subwoofer slider. To access the box below, click the advanced tab on the sound card's control panel (it may be in a different location on your sound card's control panel).

    What Should You Expect From Each of the Channels?

    In a 5.1 channel system that's properly set up, you will have strong, almost continuous output from the center channel. You will have little content in the left/right front channels unless there is something (action/dialog) happening on the far sides of the screen. The rear channels will generally have even less output. While it's possible for the left/right front/surround speakers to have high level output, it's not common. The subwoofer will be driven hard at times and at times it will be completely silent.

    Mixdown:
    If you are listening to a multi-channel program (like a DVD) but you only have 2 speakers, you may want to redirect the center channel, LFE and rear channel content to the front speakers. This can be done with the check boxes shown in the next two dialog boxes. They can be accessed by clicking the advanced button under the center and rear channel sliders in the 'all channel' mixer. Keep in mind that you probably won't get good results if you mixdown the center/sub AND choose the expander/5.1 magic options. You will have to try different settings to see what works best for your system.

     

    Note: When making changes to the various audio modes, if there isn't an immediate difference when you make a change (and particularly if there is no change when you close the dialog box where you made the change), you may have to close and restart the media player for the changes to take effect. If you are playing a CD and switch to a DVD (or vice-versa), and the audio is not as it should be, you may have to close and reopen the media player. On my system, if I'm listening to a music CD and switch to a DVD with a digital multi-channel audio soundtrack, sometimes the audio isn't right on the DVD. This can be cured by closing the media player and reopening it.

    Testing the Speaker Setup

    Virtually all sound cards include some application to allow you to send a signal to each of the output channels. The following image shows the test window for a 5.1 channel system. Generally, you can click on each speaker and a signal will be sent to that speaker (or to that particular output channel on the sound card).

    The AC-3 Filter

    To decode the digital audio data stream ,you need a decoder/filter. In this section, I'll cover many of the features of the AC-3 filter that I'm using (there may be others but, if they exist, I'm not familiar with them). This filter can be located/downloaded via Google.

    The Basics:
    This filter can convert virtually any audio format (2 channel, multichannel) to virtually any output configuration that you can imagine (within the limits of your sound card). The following is the main window for the filter. You'll see several basic controls, level indicators and various options. At this point, the most important option that you should notice is the drop-down menu that indicates that the output is set to 5.1 channels. Unless otherwise indicated, we will be dealing with a 5.1 channel system.

    Here, I expanded the menu to let you see a few of the other choices.

    Note: The filter option dialog box can be accessed in Windows Media Player by going to TOOLS >> OPTIONS >> DVD >> ADVANCED.

    These are most all of the possible choices for speaker configurations and the characteristics of each one. The number to the left of the slash is the number of front speakers. The number to the right of the slash is the number of surround channels and 'SW' indicates that it has a dedicated subwoofer output channel. In each of the different modes, the content of each channel can be set (in the matrix). I'll cover some of those options a little later.

    • 1/0       (one mono output)
    2/0       (two stereo channels)
    3/0       (three speakers across the front)
    2/1       (two stereo channels and 1 surround channel)
    3/1       (three front channels and one surround channel)
    2/2       (two front speakers and two rear surround speakers)
    3/2       (three front speakers and two surround speakers)
    1/0+SW(one mono channel and a subwoofer channel)
    2/0+SW(two stereo channels and a subwoofer channel)
    3/0+SW(two stereo channels, a center channel and a subwoofer channel)
    2/1+SW(two stereo channels, one surround channel and a subwoofer channel)
    3/1+SW(two stereo channels, one center channel, one surround channel and a subwoofer channel)
    2/2+SW(two stereo channels and two surround channels)
    3/2+SW(two stereo channels, one center channel, two surround channels and a subwoofer channel)

     

     
    The following image shows the 'mixer' page of the filter. For now, I'll only cover the basic functions.

    Master Gain:
    The gain sets the input level going into the filter. This should be left at zero (centered). If you have some source material (the audio track on a DVD) that's recorded at a very low level, you can increase it but only if the regular level controls (on the external mixers/control panels) don't provide sufficient output. When you increase the gain, be very careful. A small change will make a big difference. Most sound tracks are recorded as high as possible. Some points will be low but there will generallysome points that are at the absolute maximum output. Increasing the gain even a little could cause the signal to become distorted. If you need to increase the overall level, there are other ways to do it without creating a distorted output.

    Other Input Gains:
    The 'voice' (center/dialog channel), surround and LFE channels also set the respective levels to the input of the filter.

    Auto Gain:
    The auto gain automatically sets the master gain level so that the input to the filter does not introduce a form of clipping (overflow) in the digital section of the filter. If this option is checked and any of the input gains are set so high that distortion would be introduced, the auto gain will lower the Master gain. If you want to see it in action, you can set the 'master' level to the maximum setting and watch the 'Gain' slider continuously readjust on loud passages. You will need to have the 'one-pass Normalization un-checked for this to work as I described.

    One-Pass Normalization:
    Generally, when a digital signal is 'normalized', the volume of every part of an audio file is increased until the loudest peak is at the threshold of clipping/distortion. If this option is checked, the actual master gain will be reduced each time there is too much signal to be processed without distortion. For example, if you set the master slider to +20dB, the gain will remain at +20dB until a distortion inducing signal level is attempted to be fed into the filter. When this happens, the normalization will reduce the actual gain to a point where an equally large signal would be able to pass through the filter cleanly. The actual gain would be something lower than the +20dB level that you set. How much less depends on the level of the signal that triggered the reduction in gain. The gain will remain at the reduced level until the media player is restarted.

    Auto Matrix:
    The 'Matrix' is the block of numbers at the upper/right of the mixer dialog box. It shows the relative level of each of the signals. A value of 1 indicates that the full signal will be passed from the input to the output. A value of 0.5 means that the signal level to be passed on will be 1/2 of the input. You can enter values greater than 1 (when 'auto matrix' is defeated) but it's likely to cause problems with induced distortion on loud passages. The top row of letters is the signal from the signal source. The letters along the right side of the matrix are the output channels. L is the left signal from the source. L' is the left output. As you can see above, all of the inputs go directly to their respective outputs with no change in the signal level. This is how Dolby Digital signals will be processed (with respect to the matrix). The following are operative when the auto matrix feature is enabled.

  • Normalize Matrix
    When this is selected, the total maximum signal level through the matrix will be '1'.
  • Voice Control:
    This is activated when you want to generate a center channel from an audio program that has only two stereo channels. The output is sent to the center channel. This has no effect when the signal source is a 5.1 channel Dolby Digital signal.
  • Expand Stereo:
    This is used to derive one or two surround channels from the left and right channels of a two channel stereo signal source. This has no effect when the signal source is a 5.1 channel Dolby Digital signal. Creation of surround signals outside of digital audio only works when the source a stereo signal (mono doesn't work).

    Bass Redirect:
    I touched on this earlier. This function redirects bass from various other channels to the subwoofer. The number in the text field is the cutoff frequency for the signal being redirected. The program material that is at or below the frequency entered into the field gets redirected.

    Different Types of DVD Audio Signals

    On DVDs, you'll find several different audio programs. I'll try to cover the basics. If you are interested in more detailed information, Google can probably help.

    LPCM Audio:
    LPCM (Linear Pulse Code Modulation) is a form of digitally encoded audio similar to that used on CDs. It's low tech but it still works well. This first DVD has LPCM audio (no Dolby Digital and no DTS audio). As you can see on the input level indicators, there are only two channels (left and right). On the output indicators, you can see that all 6 output channels have signal.

    If you look at the matrix, you can see how the original left and right signals are redistributed. The left and right front outputs are 3dB down from the input level but nothing else is added to the L/R front channels. The signals to the center channel include 1/2 of each of the left and right input channels (notice that this adds up to 'one' -- it will have the highest output because all others will have an output less than 1). The surround channels are probably the most interesting (you know you're a geek when this is interesting :). As you can see both surround channels have output from each of the left and right input channels. The level is 9dB down from the input level. You should also notice that one of the values is 'negative'. What they're doing is subtracting one channel from the other. This in effect plays the difference in the two signals. For the left surround channel, the right signal is the negative channel. For the right channel, the left signal is negative. The result is relatively good and it helps to open up the soundstage, this is not substitute for a digital surround system.

    Digitally Encoded Two Channel Audio:
    The signal above was two channel audio and this is two channel audio but this two channel signal is encoded into the Dolby Digital data stream (not LPCM). If you look at the 'Decoder Info' section below, you can see that we have an AC3 data stream but the 'speakers' are listed as being Dolby Surround. This means that there is no 5.1 channel encoding. This signal is also compressed (the LPCM signal was not). The bit rate is given as 192kbps (192 thousand bits per second). This is a relatively high bit rate and will provide very good quality audio. The matrix layout would be the same as the previous example. Even though the data for the audio gets delivered in a different type of datastream, the product is the same (2 channel stereo audio).

    Dolby Digital -- AC3:
    Here, we see the first true 5.1 channel digital audio signal. Of the last few examples, this is the first in which you see all of the inputs with a signal. The matrix for this would be a series of 1s diagonally (as in the first image where we showed the matrix layout). Here, you can see that the speakers are listed as 3/2+LFE. That's a 5.1 channel system. As you can see, the bitrate is higher. Some will argue that the sound quality will be better at 448kbps (vs 192kbps) but I can't hear the difference between the two.

    DTS:
    If you ask some people, DTS (Digital Theater Systems) is the cream_of_the_crop. As you can see, the bit rate is even higher than the Dolby Digital/AC3 bitstream. Again, there's no way that I would be able to hear the difference. DTS is less popular than Dolby Digital for at least a couple of reasons. One is that Dolby Digital is the 'standard' when it comes to audio data on DVDs. If a DVD has DTS, it also has Dolby Digital. Since the Dolby Digital audio is going to be considered to be the default audio stream, you may have to actively choose the DTS audio in one of the menus. Another reason DTS may be less popular is that it takes up more space on the DVD. The higher bitrate simply takes more storage space (all else being equal).

      

     
 
 
 
 
 

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