JPS6349952B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses pulse code modulation (PCM) to generate an audible signal tone.
This invention relates to a digital tone signal generator that transmits codes as they are.

Conventionally, as a pulse code modulated digital tone generation method, a certain section length of a tone signal is sampled every sampling period, encoded, and written in a read-only memory (ROM) and read out periodically to generate a tone signal. A method of outputting is known. An example of this configuration is shown in FIG.
In the figure, 10 is a frame clock input terminal;
20 is a tone signal output terminal, and 30 is a parity check output terminal. Furthermore, 3 is a signal ROM in which a plurality of 8-bit PCM samples obtained by sampling and encoding tone signals are written. this
The ROM is addressed by a frame counter 1 which is clocked every PCM frame, and the time axis direction is controlled. The output code from the ROM is converted into a serial PCM stream by the parallel/serial conversion circuit 5. On the other hand, depending on the purpose of use, tones may be required to be continuous or intermittent. In this case, the PCM stream is gated by the output of the on/off counter 6, which produces an intermittent tone.
Get PCM stream. Generally, in such a tone signal generation circuit, a method of performing a memory parity check has been conventionally used in order to ensure reliability as a system. As is well known, the PCM code consists of 8 bits. On the other hand, the reality
The length of the ROM in the bit direction is usually a power of two, for example 4 bits or 8 bits. Therefore, in order to perform a parity check, a 1-bit ROM2 for parity is added separately from the 8-bit ROM3 for signals, as shown in Figure 1, and this 2-bit ROM2 is added.
The parity check was carried out by drawing the various ROM outputs into the parity check circuit 4. In this manner, in the prior art, system reliability has been ensured by adding a ROM for parity in addition to the ROM for storing tone signals and performing a parity check. However, ROMs generally have the disadvantage of high power consumption, which increases the running cost of the equipment, and in telephone switching systems that require a large number of such ROMs, parity
Adding ROMs was never the preferred method.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide an economical digital tone signal generator.

The gist of the present invention is to forcibly use the least significant bit (LSB) of the 8-bit PCM code as the parity bit. As shown in Figure 3, the 8-bit PCM format uses 1 bit to represent the code,
The next 3 bits are defined to represent the segment number (0-7), and the following 4 bits are defined to represent the intra-segment level (0-15). Normally, it is necessary to adhere to this regulation and code the signal level. When a single frequency signal is encoded correctly and according to regulations, it is guaranteed that the signal-to-quantum noise ratio will usually be greater than 35 dB.
It is also ensured that harmonic distortion noise of the signal is sufficiently reduced. However, since the tone signal is an audible signal that the telephone user temporarily hears, the above-mentioned slight increase in noise does not pose a problem at all. Therefore, we decided to use the LSB of 8-bit PCM while being aware of the increase in noise.
is the parity bit that is satisfied by the entire 8-bit code.

Furthermore, when considering the reliability of the entire system, it is better to perform a parity check on the PCM code, including when the tone signal is interrupted. For this reason, when the tone signal is cut off, the PCM of the tone is not simply gated, but a PCM pattern that is modified within the range where the parity condition is satisfied with the zero level of the signal suitable for the cutoff of the tone is configured to be sent out. The present invention will be explained in detail below.

FIG. 2 shows an embodiment of the present invention. 3 is a signal ROM that stores samples of tone signals. Of course, as mentioned above, it is assumed that the LSB of the 8-bit PCM has been transformed so as to satisfy the parity condition as a whole. As in the case of the conventional column, this signal ROM is addressed by a frame counter 1 which is clocked every sample period (typically 125 μζ) and whose output is converted into a serial PCM stream by a parallel to serial converter 5. On the other hand, a selector 9 is controlled by the output of the counter 6 which produces an intermittent tone signal. The first input of this selector is the aforementioned PCM stream, and the second input is a PCM pattern representing the zero level of the tone signal.
Of course, this pattern also satisfies the parity condition. For example, the patterns for the A-method and the μ-method under the odd parity condition are shown in Figure 4.
It is shown in Figure 5. A PCM stream of intermittent tone signals is obtained from the output of the selector 9.
4 is a circuit that performs a parity check every 8 bits on a serial PCM stream, and the test result is outputted to a terminal 30.

As can be seen from the above explanation, by including the parity bit within the 8-bit PCM code itself, the present invention eliminates the need to add a ROM for the parity bit. In other words, it has become possible to reduce the number of ROM1 chips. In the explanation of FIG. 2, only one type of tone generation is shown for simplicity, but in a typical telephone switching system, ten or more types of tones may be required. In this case
The effect of reducing the ROM was extremely large, and an economical tone signal generator became possible.

Since the parity condition is satisfied for an 8-bit PCM code, including tone interruptions, the parity check can be performed anywhere in the telephone switching system (focusing on the tone time slots). This helps ensure higher reliability of the system as a whole.

Although the embodiments of the present invention have been described above with respect to tone generators, it goes without saying that the gist of the present invention can also be applied to so-called multi-frequency signal transmitters. In this case, it goes without saying that it can be put to practical use by paying attention only to the harmonic noise that increases to some extent.

[Brief explanation of drawings]

Figure 1 is a diagram of a conventional tone signal generation system;
The figure is a diagram of the tone signal generation method according to the present invention, Figure 3 is a diagram showing the format of the PCM code, Figure 4 is
FIG. 5 is a diagram showing zero level PCM patterns in the A-method and the μ-method, respectively. 1: Frame counter, 3: Signal ROM,
4: Parity check circuit, 5: Parallel-serial conversion circuit,
6: On-off counter, 8: Pattern generation circuit, 9: Selector, 10: Frame clock input terminal, 20: Tone signal output terminal, 30: Parity check output terminal.

Claims (1)

[Claims] 1. A read-only memory that stores a plurality of sample values representing a tone signal encoded by pulse code modulation (PCM); a tone signal generating device comprising means for generating a tone signal by reading the sample value from the memory, the tone signal generating device having a data structure in which the least weighted bit (LSB) of each sample value to be written in the memory is set as a parity bit. . 2. The tone signal generating device according to claim 1, wherein a code pattern satisfying a parity condition is output in place of the memory output when the intermittent tone signal is interrupted, and the tone output is subjected to a parity check.