Before you start
Objectives: Learn what is sound card, how does it work, different audio technologies that it utilizes, and which connectors can it have.
Prerequisites: no prerequisites.
Key terms: audio, digital, analog, sample, wave, channel, standard, dolby, bit, hz, connector, interface
The task of a sound card in a PC system is to convert digital data from the PC into analog audio that can be reproduced on speakers. The component which does this is called the Digital to Analog Converter (DAC). To input sound to our PC we have to convert analog sound to the digital sound. The component which does this is called the Analog to Digital Converter (ADC). Another component of the sound card which participates in the whole process is the Digital Signal Processor (DSP). DSP is an on-board processor that reduces the CPU load. To understand all this we first need to know how sound waves work. Sound waves are propagated through the air using compression waves. At the origin the sound wave is created by compressing and uncompressing the air. Natural sound wave is an analog wave. When we’re dealing with an analog wave we need to be aware of two things that describe it. The first is the frequency. The frequency is the distance between the tops (peaks) or the valleys of the wave.
Image 324.1 – Frequency
The second thing is the amplitude or volume. Amplitude is the total height of the wave.
Image 324.2 – Amplitude
The thing is, PCs deal with digital signals. In order to produce or capture a sound wave with a PC we have to take analog wave and represent it digitally. To do that we have to sample it. That means that we take a look at the wave at some point and note the amplitude and the frequency of the wave at that particular point. We do that trough the whole wave on many points. By doing that we create a digital approximation of the analog wave.
Bits Per Sample
When we are sampling sound waves there’s two different parameters that we need to keep in mind. The first one is the number of bits of memory used to represent a given sample (bits per sample). The more bits we use to represent a given sample the closer digital sample approximates the analog wave. For example, we can create an eight bit sample of the wave at any given point. If we use 8 bits we can get 256 different decimal numbers (2 to the power of 9) to represent a particular sample. The range starts from 0, so we have numbers from 0 to 255. Those are the numbers that can be used in decimal form to represent the sample. If we use 16 bits to represent the sample, we have up to 65536 possible values to represent the wave at any given point (2 to the power of 16). Note that values start from zero so the range is from 0 to 65535. If we go further, the 20 bit sound cards use a sampling size of 1048576, 24 bit sound cards use a sampling size of 16777216, 32 bit sound cards use a sampling size of 4294967296, etc.
The second parameter we need to keep in mind when converting analog to digital waves is the sampling rate. The sampling rate is the number of times per second that a sample is taken (the number of analog signal samples taken over a period of time). The more samples we take per second the closer the digital wave is to the analog wave. Sample rates are expressed in cycles per second, called hertz. For example, sampling rate could be 1000 hertz (Hz) which is 1 kilohertz (kHz). For example if we use 8 kilohertz sampling rate, the sound quality would be really bad, it would sound like a telephone connection. Actually 8 kHz is adequate for telephone conversation because the human voice range is about 4 kHz, but for PC audio this is not nearly enough. 22 kHz is the radio quality. The 44 kHz is the sample rate that can reproduce the audio frequencies up to 20,500 hertz, which covers the full range of human hearing. 44 kHz is considered CD quality. 48 kHz is used in Digital TVs and DVD movies. 96 kHz is used in DVD audio. 192 kHz is used by LPCM (Linear Pulse Code Modulation) which is a DVD-music production format. It is also used in BD-ROM (Blu-ray Disc-ROM) and HD-DVD (High-Density-DVD), which are two optical disc formats providing HD video and high density data storage. Higher sample rates also require more bits of data per sample (more memory). For example, to have CD quality audio we also have to use 16 bits per sample.
Audio File Size
We can sample the wave more frequently but the downside with this is that the digital files that we create using higher sampling rate and more bits per sample, get really big. That’s because we use more samples and more memory to store those samples. Because of that different compression schemes have been invented to reduce the size of digital audio files maintaining the fidelity of the original sound from the higher sampling rate. For example we have MPEG files, Microsoft Windows Media Files, Advanced Audio Coding, etc. These schemes use various types of compression to shrink the size of digital files. When talking about audio formats we should mention the there major groups of audio file formats. We have uncompressed audio formats, such as WAV (Windows standard), AIFF (Audio Interchange File Format, the Macintosh equivalent of the WAV), AU or raw header-less PCM (UNIX standard, supported by most Web browsers). Next, we have formats with lossless compression, such as FLAC, Monkey’s Audio (APE), WavPack (WV), TTA, ATRAC Advanced Lossless, Apple Lossless (m4a), MPEG-4 SLS, MPEG-4 ALS, MPEG-4 DST, Windows Media Audio Lossless (WMA Lossless), and Shorten (SHN). In the end, we have formats with lossy compression, such as MP3 (MPEG-1 Layer 3), Vorbis, Musepack, AAC (Advanced Audio Coding, also known as MPEG-2), ATRAC, Windows Media Audio Lossy (WMA lossy), RA or RM (Real Networks, developed for streaming audio files).
Sound Card Considerations
A basic sound card will have analog speaker-out jack and an analog microphone jack. This allows us to send audio out to the speakers, as well as input audio through the microphone. Keep in mind that the speaker-out signal is amplified and that the computer controls the sound level that is sent. Some cards will also have a line-out connector, which is used to send audio to other sound devices. Keep in mind that the line-out signal is unamplified. Most sound cards will also have a line-in connector which allows us to pull audio from other sound devices (CD players, musical instruments, etc) and bring to our PC to record it, manipulate it or simply play it on our PC. All interfaces mentioned so far are analog, and a large number of devices will use them. However, newer devices can process digital audio signals. Consumer standard for digital audio transfer is S/PDIF, which stands for Sony/Phillips Digital Interface. It’s used to transfer digital files from one system to another, without having to go through a digital to analog and analog to digital conversion. Instead of that the sound is transferred digitally. Some sound cards also have a Midi/joystick port, which we use to connect a game controller to our PC or to interface with Midi sound devices. Some sound cards will also include HDMI ports.
Number of Channels
Different sound cards will use different number of channels. Channels can make sound more realistic. Basic sound cards will have 2 channels, which isstereo audio. Standard radio and TV have stereo sound. Really old sound cards actually had one channel (mono sound), but today all sound cards will have at least two channels. We can also purchase a 4 channel stereo audio card, which gives us front left, front right, rear left and rear right channel. 4 channel audio is quadraphonic audio, and it is an early attempt of surround sound. Higher quality sound cards will also implement Dolby Digital Sound. Dolby Digital is a digital audio coding technique that reduces the amount of data needed to produce high quality sound. It also reduces noise coming through the system. Dolby Audio provides 5 full bandwidth channels (also called audio channels), which are front left, front right, center, surround left and surround right. Dolby Digital also includes one low bandwidth channel called the Low Frequency Effect or LFE channel (also called the subwoofer channel or effects channel). Together, this Dolby Digital Standard is called Dolby 5.1 because we have five full bandwidth channels, and one LFE channel. All channels are delivered on 6 strategically placed speakers. There are other Dolby Digital implementations like the Dolby Digital 6.1 and Dolby Digital 7.1. The 6.1 and 7.1 version of Dolby Digital add more channels. For example, 6.1 has 6 audio channels and 1 effects channel. 7.1 has 7 audio channels and 1 effects channel.
Some sound cards will support Digital Theater System or DTS standard. This standard is designed to produce near theater quality audio from your PC system. It has 6 channels and it’s based on Dolby 5.1. In order to produce DTS sound we also have to have a DTS compatible speakers. Some sound cards will support the EAX Standard. The EAX standard is designed to produce interactive 3D audio in PC gaming. There is also a THX standard which is a a sound quality standard, originally created for film, but now it is available on sound cards. Some cards will support MIDI (Musical Instrument Digital Interface). This is a protocol for recording and playing audio created on digital synthesizers.
Almost all sound cards will have ports which accept 3.5 mm plugs for analog input and output. We call those ports Mini TRS ports. The number of Mini TRS ports will depend on the number of speaker channels, microphone or line in support, etc. Mini TRS use standardized color coding which can help us determine the proper port. For example, Mic-in is pink, line-in is blue, and line-out is green (front speakers). We can also have other colors. For example black port is usually for rear speakers, and orange is for center and surround speakers.
Image 324.3 – Mini TRS Ports
Image 324.4 – Mini TRS Male Plug (3.5 mm)
Although these colors are standard, we should consult the sound card documentation for specific details. There is also a Toslink connector which is used with digital optical input/output, for S/PDIF audio.
Image 324.5 – TOSLINK Male Plug
We also have an RCA connector which is usually used for coaxial digital input or output for S/PDIF audio. It can be used for analog audio, but RCA connector on a sound card is normally used for S/PDIF digital audio.
Image 324.5 – RCA Male Plug
A DB-15 connector on a sound card is used to connect to MIDI devices or game joysticks. Sometimes it is called a game port.
Image 324.6 – MIDI/Game Port
A sound card with an HDMI port is capable of sending HD audio to an HDMI device.
Image 324.7 – HDMI Port
The sound card converts an analog signal to a digital signal (for example from microphone) or a digital signal to an analog signal (sent from PC to speakers). When we are sampling sound waves there’s two different parameters that we need to keep in mind. The first one is the number of bits of memory used to represent a given sample (bits per sample). The second parameter is the sampling rate. Using higher sampling rate and more bits per sample means better audio quality, but it also means bigger audio files. Different sound cards will use different number of channels. Typical connectors which can be found on sound cards are Mini TRS, TOSLINK, RCA, DB-15, and HDMI.