Microsoft KB Archive/75632

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Anatomy of the Sound Blaster

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Q75632

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) 3.0

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SUMMARY
This article is a brief overview of the Sound Blaster card hardware and how it works with the Multimedia Extensions to Microsoft Windows.

MORE INFORMATION
The following is a list of features that the Sound Blaster card offers. Differences between the version 1.0 and version 1.5 boards are noted where appropriate. Differences between the versions 1.x and version 2.0 digital signal processors (DSPs) are also noted.

  12-Voice Stereo C/MS Music C/MS stands for creative music synthesizer. C/MS is a low-end synthesizer that is compatible with the Game Blaster. As the name implies, it synthesizes music by taking a basic waveform and manipulating it (note that this is a generalization).

This particular synthesizer uses a technique called square wave synthesis. It is similar to a synthesizer found in an inexpensive toy; but the Sound Blaster does not have a keyboard attached. Instead using of keys, notes are turned on and off by writing commands to hardware registers on the Sound Blaster board.

When C/MS is marketed as having 12 voices, reference is being made to the number of different sounds the C/MS synthesizer can play at the same time (commonly referred to as &quot;polyphony&quot;).

The Sound Blaster uses two C/MS chips, each with six voices. In this way, the first chip can be assigned to the left channel and the second chip can be assigned to the right channel. Thus, the Sound Blaster can produce stereo sound.

The version 1.0 Sound Blaster board was shipped with the C/MS chip set installed. However, Creative Labs removed the chips from the version 1.5 board and sells the C/MS chips separately. The version 1.0 box sells the Sound Blaster as the &quot;24-Voice All-in-One Sound Card.&quot; The version 1.5 box shortened this to &quot;The All-in-One Sound Card&quot; because they removed 12 of the &quot;voices.&quot;

To determine if the C/MS chips are in place, run TEST-SBC.EXE. If the chips are present, the features list will list them. The Multimedia Extensions to Windows currently does not use the C/MS feature.   11-Voice FM Music (AdLib Compatible) FM stands for frequency modulation, which is how the sound is produced. The Sound Blaster has 11 FM-voices (11 notes polyphony), and unlike FM-stereo radio, the sound is mono only. FM-synthesis produces slightly better sounds than square wave synthesis, which is what the C/MS chip set does.

The Sound Blaster's FM synthesizer is &quot;AdLib&quot; compatible. AdLib is the brand name of an FM synthesizer chip set; many different companies (including Creative Labs) clone these chips. The AdLib driver in Multimedia Windows will work with any AdLib-compatible synthesizer, which, of course, includes the Sound Blaster.   1 Digitized Voice Channel This can be called many things: sampled sound, digitized sound, and so forth. The Sound Blaster refers to it as a digitized voice, which is not the same as the voices mentioned above. The Multimedia Extensions to Windows calls this digitized sound output &quot;waveform output,&quot; which is confusing because all sound is comprised of waves! Digital enthusiasts call this pulse code modulation (PCM), which is actually a more accurate title.

Digitized sound is real sound (human voice, music, effects, and so forth) that is sampled and digitized with a digital-to-analog converter (DAC). This is the same type of data stored on audio compact discs (CDs); however, CDs have a much greater resolution both in sample rate (44.1 KHz) and sample word size (12 to 16 bits).

PCM is a simple technique of encoding analog audio in a digital format, where the waveform amplitude is sampled periodically (at the sampling frequency) and stored as a number. The term &quot;8 bit&quot; means that the stored number can have one of 256 discrete values; 12 bit has 4096 discrete values; and 16 bit has 65535.

The Sound Blaster has one mono 8-bit DAC that can be set to a sample rate between ~4 KHz and ~23 KHz. This does not produce very high quality but it can be fun none the less. Incidentally, this is where that 24th voice of the version 1.0 card came from: 12 C/MS + 11 FM + 1 DAC = 24.

The Multimedia Extensions require a minimum of 11 KHz and 22 KHz 8-bit wave output, which the Sound Blaster can do.

Another interesting thing about the Sound Blaster is that it supports three different adaptive differential pulse code modulation (ADPCM) decompression ratios (as well as no compression) in hardware. Below is a list of these ratios:

 2 to 1 data compression: 4 bit ADPCM 3 to 1 data compression: 2.6 bit ADPCM 4 to 1 data compression: 2 bit ADPCM

This may be the only justification for Creative Labs's use of the term digital signal processor (DSP) for its programmable logic array (PLA). Keep in mind that the greater the ratio, the greater the degradation in sample quality.

The Multimedia Extensions to Windows does not make use of ADPCM at this time. Also note that Multimedia Windows uses the direct memory access (DMA) mode of the Sound Blaster for recording and playing digitized sound.

If a board with the versions 1.x DSP is installed and Multimedia Windows is running in enhanced mode, a periodic click is audible when playing a wave file. This is caused by interrupt latency, meaning that interrupts are not serviced immediately. This causes the Sound Blaster to click because the versions 1.x DSP produce an interrupt when the current DMA buffer is exhausted. The click is the time it takes for the interrupt to be serviced by the Sound Blaster driver (which is delayed by the 386 enhanced mode of Windows).

The click is still present in standard mode, although it is much less pronounced because the interrupt latency is less. The click is more pronounced for pure tones.

The version 2.0 DSP solves this problem by using the auto- initialize mode of the DMA controller (the 8237). In this mode, the DMA controller automatically reloads the start address and count registers with the original values. In this way, the Sound Blaster driver can allocate a 4K DMA buffer; using the lower 2K as the &quot;ping&quot; buffer and the upper 2K as the &quot;pong&quot; buffer.

While the DMA controller is processing the contents of the ping buffer, the driver can update the pong; and vice versa. Therefore, when the DMA controller auto-initializes, it will already have valid data available. This removes the click from the output sound.

However, the 386 enhanced mode of Windows 3.0 does not support the auto-initialize mode of the DMA controller. Therefore, Multimedia Windows provides a virtual device driver (VxD) called VADMAD.386 (virtual auto-initialize DMA device) that attaches itself to the virtual DMA device (VDMAD). Its purpose, as the name implies, is to support the auto-initialize mode of the DMA controller. However, VADMAD.386 does not fully support this mode -- it was designed only to correct the problem with the Sound Blaster. Because Windows 3.1 supports auto-initialize DMA, do not use VADMAD with Windows 3.1.   Voice Input (Digital Sampling) Capability This is more commonly referred to as a digitizer. The Sound Blaster supports an 8-bit analog-to-digital converter (ADC) with a variable sampling rate of ~4 KHz to ~12 KHz. The Multimedia Extensions to Windows requires a minimum sampling rate of 11 KHz. This provides sound quality approximately equal to that of normal voice phone lines.

The Sound Blaster has a built-in microphone jack and amplifier with auto-gain control (AGC) for direct input (sampling) of sound. The microphone used should be ~600 ohms; AGC range 10 mV to 100 mV. This translates to &quot;an inexpensive, low-end microphone.&quot; </li>  Built-In Stereo Power Amplifier The Sound Blaster has a built-in amplifier that provides a maximum output of 4 watts per channel into a 4 ohm load. This is enough to drive an inexpensive set of headphones or some &quot;Walkman&quot; style speakers.

And although the output is &quot;stereo,&quot; the ONLY thing that is truly stereo on the Sound Blaster is the C/MS chip set (which Multimedia Windows does not use). All of those neat sounds played in Multimedia Windows are mono. </li>  Built-in Game I/O Port This is a standard 15-pin D-Sub PC analog joystick adapter port. The Sound Blaster allows two joysticks to be connected through this port with the aid of a Y-cable (sold separately). The Multimedia Extensions to Windows can be configured to use this as two normal two-dimensional joysticks or one three-dimensional joystick.

The only confusing part is Creative Lab's use of &quot;I/O&quot; for this joystick port. Joysticks are input devices only. The only output that this port provides is for MIDI (see part 7 below); this must be what Creative Labs was referring to. </li>  Built-in MIDI Interface MIDI stands for musical instrument digital interface. Simply put, this is a protocol for connecting intelligent musical instruments and computers for the purpose of controlling the instruments in real time.

To use the MIDI interface on the Sound Blaster, the MIDI connector box is required (sold separately). It plugs into the joystick port and has a pass-through for the joystick (so the joystick remains available). The MIDI connector box gives one MIDI-in and five MIDI-out connectors.

With the version 1.x DSP, the MIDI port can be used only in half-duplex mode (one direction at a time). The version 2.0 DSP allows full-duplex operation, which allows playing and recording simultaneously (with the option of other accompanying instruments).

The MCI-sequencer in Multimedia Windows allows use of the MIDI port or the AdLib (FM) synthesizer to play MIDI data. An external synthesizer will probably sound much better than the AdLib synthesizer and is therefore recommended. </li></ol>

Additional query words: 3.00 soundblaster

Keywords : kbmm

Issue type :

Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300