Adaptive Delta Modulation – Block Diagram and Applications

In communication systems, the modulation methods are used to transmit signals over long distances. In the modulation process, the properties of a high-frequency signal such as amplitude, phase, etc… are changed according to the low-frequency base-band signal. With the increase in digital technology and advancements in signal processing techniques demand of digital communication increased. Many methods for Digital – to- analog and Analog-to-digital conversion of sampled signals were introduced for digital communication. Pulse code modulation, Differential pulse code modulation, Delta modulation, and Adaptive Delta Modulation are popular methods used for signal processing in digital communication. In this article let us take a look at Adaptive Delta Modulation method.

What is Adaptive Delta Modulation?

This Modulation is the refined form of delta modulation. This method was introduced to solve the granular noise and slope overload error caused during Delta modulation.

This Modulation method is similar to Delta modulation except that the step size is variable according to the input signal in Adaptive Delta Modulation whereas it is a fixed value in delta modulation.

Block Diagram

The transmitter circuit consists of a summer, quantizer, Delay circuit, and a logic circuit for step size control. The baseband signal X(nTs) is given as input to the circuit. The feedback circuit present in the transmitter is an Integrator. The integrator generates the staircase approximation of the previous sample.

At the summer circuit, the difference between the present sample and staircase approximation of previous sample e(nTs) is calculated. This error signal is passed to the quantizer, where a quantized value is generated. The step size control block controls the step size of the next approximation based on either the quantized value is high or low. The quantized signal is given as output.

At the receiver end Demodulation takes place. The receiver has two parts. First part is the step size control. Here the received signal is passed through a logic step size control block, where the step size is produced from each incoming bit. Step size is decided based on present and previous input. In the second part of the receiver, the accumulator circuit recreates the staircase signal. This waveform is then applied to a low pass filter which smoothens the waveform and recreates the original signal.

Adaptive Delta Modulation Theory

In Adaptive Delta Modulation, the step size of the staircase signal is not fixed and changes depending upon the input signal. Here first the difference between the present sample value and previous approximation is calculated. This error is quantized i.e. if the present sample is smaller than the previous approximation, quantized value is high or else it is low. The output of the one-bit quantizer is given to the Logic step size control circuit where the step size is decided.

At the logic step size control circuit, the output is decided based on the quantizer output. If the quantizer output is high, then the step size is doubled for the next sample. If the quantizer output is low, the step size is reduced by one step for the next sample.

Advantages

Some of the advantages of this modulation method are listed below-

• Adaptive delta modulation decreases slope error present in delta modulation.
• During demodulation, it uses a low pass filter which removes the quantized noise.
• The slope overload error and granular error present in delta modulation are solved using this modulation. Because of this, the signal to noise ratio of this modulation is better than delta modulation.
• In the presence of bit errors, this modulation provides robust performance. This reduces the need for error detection and correction circuits in radio design.
• The dynamic range of Adaptive delta modulation is large as the variable step size covers large range of values.

Differences between Delta Modulation and Adaptive Delta Modulation

The differences between adaptive delta modulation and delta modulation are listed below-

• In Delta Modulation step size is fixed for the whole signal. Whereas in Adaptive delta modulation, the step size varies depending upon the input signal.
• The slope overload and granular noise errors which are present in delta modulation are not seen in this modulation.
• The dynamic range of Adaptive delta modulation is wider than delta modulation.
• This modulation utilizes bandwidth more effectively than delta modulation.

Applications

Some of the applications of this modulation method are listed below-

• This modulation is used for a system which requires improved wireless voice quality as well as speed transfer of bits.
• In television signal transmission this modulation process is used.
• This modulation method is used in voice coding.
• This modulation is also used as a standard by NASA for all communications between mission control and spacecraft.
• Motorola’s SECURENET line of digital radio products uses 12kbits/sec Adaptive Delta Modulation.
• To provide voice detection quality audio at deployed areas, military uses 16 to 32 kbit/sec modulation system in TRI-TAC digital telephones.
• US army forces use 16kbit/sec rates to conserve bandwidth over tactical links.
• For improved voice quality US Air Forces uses 32kbits/sec rates.
• In Bluetooth-services to encode voice signals, this modulation is used with 32bits/sec rates.
• HC55516 decoder is used in various arcade games such as sinistar and smash tv and pinball machines such as gorgor or space shuttle, to play pre-recorded sounds.
• Adaptive delta modulation is also known as continuously variable slope delta modulation.

This modulation encodes at 1-bit per sample. Here the encoder maintains a reference sample and a step size.  Before deciding the step size of the input signal it is compared with the reference sample. This modulation method compromise between simplicity, low bitrate, and quality.

This modulation method was first published by Dr. John E. Abate in 1968 at NJ Institute of technology. By using this modulation method many minute details of the signal can be preserved. Thus, this modulation method provides good quality output in addition to fast encoding. This modulation is the first step in the conversion of the analog signal to a digital signal. The next step is the representation of this digital signal in mathematical form, for which digital multiplexing techniques are introduced. Adaptive Delta Modulation is also known as?