Audio Circuits

There are two audio circuits for generating sound. They are identical but completely independent and can be operated simultaneously to produce sound effects through the TV speaker. Each audio circuit has three registers that control a noise-tone generator (what kind of sound), a frequency selection (high or low pitch of the sound), and a volume control. Each audio circuit consists of parts described below, and in figure 7.


The noise-tone generator circuit contains a nine bit shift counter which may be controlled by writing to the 4 bit audio control registers (AUDC0, AUDC1). It is clocked by the frequency select circuit. The values written cause different kinds of sounds to be generated. Some are pure tones like a flute, others have various "noise" content like a rocket motor or explosion. Even though the TIA hardware manual lists the sounds created by each value, some experimentation will be necessary to find "your sound". The control registers can be modified at any time to select different shift counter feedback taps and count lengths to produce a variety of noise and tone qualities.


Frequency selection is controlled by writing to a 5 bit audio frequency register (AUDF0, AUDF1). Clock pulses (at approximately 30 KHz) from the horizontal sync counter pass through a divide by N circuit which is controlled by the output code from a five bit frequency register (AUDF). This register can be loaded (written) by the microprocessor at any time, and causes the 30 KHz clocks to be divided by 1 (code 00000) through 32 (code 11111). This produces pulses that are digitally adjustable from approximately 30 KHz to 1 KHz and are used to clock the noise-tone generator. By combining the pure tones available from the noise-tone generator with frequency selection, a wide range of tones can be generated.


Volume is controlled by writing to a 4 bit audio volume register (AUDV0, AUDV1). Writing 0 to these registers turns sound off completely, and writing any value up to 15 increases the volume accordingly. The shift counter output is used to drive the audio output pad through four driver transistors that are graduated in size. Each transistor is twice as large as the previous one and is enable by one bit from the audio volume registers. The audio volume registers may be modified at any time. As binary codes 0 through 15 are loaded, the pad drive transistors are enabled in a binary sequence. The shift counter output therefore can pull down on the audio output pad with 16 selectable impedance levels.

Figure 7

SECAM Compatibility

SECAM machines use PAL software with one exception. when a sound is to be turned off, it must be one by setting AUDV0/AUDV1 to 0, not by setting AUDC0/AUDC1 to 0. Otherwise, you get an obnoxious background sound.

see TIA