Audio Circuit DiagramsSignal Generators

Speech/Sound Memory

OKI’s Series MSM6372-6375 enable the reproduction of speech or other sound stored in their internal ROM. The capacity of the ROM lies between 128 kbit and 1024 kbit, depending on the type. The type and the sampling frequency specified (4 kHz, 6.4 kHz or 8 kHz) determine the length of the stored speech, which is 4-64 seconds. This time may be divided into  111 words that can be addressed individually.

Each IC contains a 12-bit digital to analogue converter (DAC) and a fourth order low pass filter. The customer can specify the words that must be stored on the mask-programmable ROM. As an example, the Type MSM6374-007 is programmed to tell the time in English.

Speech or Sound Memory schematic diagram

Based on this IC, the circuit shown in Fig, 1 is intended to be connected to the I2C I/O card (see Ref.1) of which the two converters have been omitted. The card then serves as an interface between the I2c  bus and IC2.

The wanted word is selected via inputs 10-16. Since the MSM6374-007 contains only words at addresses where 13 is zero, the corresponding input is strapped to earth. This arrangement leaves two of the eight available I/O bits for starting and timing of the words. These bits are available at st (start input) and NAR (next address request output). Briefly, the control is

  • Wait until NAR is high;
  • Key in the address;
  • Wait not less than 10 seconds;
  • Briefly render ST low ( pulse duration 0.35-350 us).

Normally the nar signal indicates that the next address may be keyed in before the entire word has been spoken. This arrangement provides a smooth transition between words or parts of words.

To simplify the control of the synthesizer, the start pulse is not generated by the software, but by monoflop IC1a. This stage is triggered by both the first and last transition of the start signal, which is applied to the trigger inputs via differentiating networks R5-C7 and R7-C8. The timing diagram in Fig. 2 shows what happens. Every time the computer writes data to IC2, the software inverts bit P6. It then takes 30 us before the onset of the start signal, which is 250 us long. In this way, the control computer would have to write the address first, then, after 10 us, the start signal, and finally end the start signal.

Resistors R2, R3, and capacitor C3 set the oscillator frequency to 64 kHz, resulting in a sampling frequency of 6.4 kHz.

Resistors R4 and capacitor C5 provides a power-up reset.

Capacitor C4, which forms part of the output filter, provides an improvement of the signal to noise ratio.

A second channel enables the IC to give a speech with echo, two tones, or a tone with three different volumes. This facility cannot be used in this application owing to lack of I/O bits. Input 2CH is, therefore, disabled by strapping it to the positive supply line.

Speech or Sound Memory schematic diagram

The output signal is raised to about 1 W into 8 (ohm) by integrated bridge amplifier IC3. The circuit may be controlled via the computer to I2C interface by software that once installed, may be enabled by a suitable key combination. It is possible for either the time to be spoken or an alarm time to be set. The use of this software presupposes that the I2C driver(Type 1671*) has been installed. When program 1773* is run, it installs itself, after which it is enabled (even if another program is being run) by the simultaneous pressing of keys CTRL and F1. If that combination cannot be used, for instance, because it is used by another program, another combination may be chosen by loading the program with TALKTIME/H. You will then be asked to key in the alternative combination. Note that in the combination the program can recognize only the lefthand shift key.

Speech or Sound Memory schematic diagram

When the combination has been keyed in, a self-evident menu appears after about a second, provided the screen is not in the graphics mode. In that case, a high tone is emitted, whereupon only functions tell time and alarm on/off are available.

The circuit draws a current of not more than 300 mA.

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