An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively) high-frequency
amplitude modulated
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to t ...
signal as input and provides an output, which is the
demodulated ''
envelope
An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.
Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a ...
'' of the original signal.
Circuit operation
The
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
in the circuit above stores charge on the rising edge and releases it slowly through the
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...
when the input signal amplitude falls. The diode in series
rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal.
General considerations
Most practical envelope detectors use either half-wave or full-wave
rectification
Rectification has the following technical meanings:
Mathematics
* Rectification (geometry), truncating a polytope by marking the midpoints of all its edges, and cutting off its vertices at those points
* Rectifiable curve, in mathematics
* Recti ...
of the signal to convert the
AC audio input into a pulsed
DC signal.
Filtering
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...
is then used to smooth the final result. This filtering is rarely perfect and some "ripple" is likely to remain on the envelope follower output, particularly for low frequency inputs such as notes from a bass instrument. Reducing the filter cutoff frequency gives a smoother output, but decreases the high frequency response. Therefore, practical designs must reach a compromise.
Definition of the envelope
Any
AM or
FM signal
can be written in the following form
:
In the case of AM, φ(''t'') (the phase component of the signal) is constant and can be ignored. In AM, the
carrier frequency
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
is also constant. Thus, all the information in the AM signal is in ''R''(''t''). ''R''(''t'') is called the ''envelope'' of the signal. Hence an AM signal is given by the function
:
with ''m''(''t'') representing the original audio frequency message, ''C'' the carrier amplitude and ''R''(''t'') equal to ''C'' + ''m''(''t''). So, if the envelope of the AM signal can be extracted, the original message can be recovered.
In the case of FM, the transmitted
has a
constant envelope ''R''(''t'') = R and can be ignored.
However, many FM receivers measure the envelope anyway for
received signal strength indication
In telecommunications, received signal strength indicator (RSSI) is a measurement of the power present in a received radio signal.
RSSI is usually invisible to a user of a receiving device. However, because signal strength can vary greatly and af ...
.
Diode detector
The simplest form of envelope detector is the
diode detector which is shown above. A diode detector is simply a
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diod ...
between the input and output of a circuit, connected to a resistor and capacitor in parallel from the output of the circuit to the ground. If the resistor and capacitor are correctly chosen, the output of this circuit should approximate a voltage-shifted version of the original (
baseband
In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable i ...
) signal. A simple filter can then be applied to filter out the DC component.
Precision detector
An envelope detector can also be constructed using a
precision rectifier
The precision rectifier is a configuration obtained with an operational amplifier in order to have a circuit behave like an ideal diode and rectifier.Paul Horowitz and Winfield Hill, ''The Art of Electronics''. 2nd ed. Cambridge University Press, ...
feeding into a
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
.
Drawbacks
The envelope detector has several drawbacks:
* The input to the detector must be
band-pass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-port ...
ed around the desired signal, or else the detector will simultaneously demodulate several signals. The filtering can be done with a tunable filter or, more practically, a
superheterodyne receiver
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original car ...
* It is more susceptible to noise than a
product detector
* If the signal is
overmodulated(i.e.
modulation index The modulation index (or modulation depth) of a modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called ...
> 1), distortion will occur
Most of these drawbacks are relatively minor and are usually acceptable tradeoffs for the simplicity and low cost of using an envelope detector.
Demodulation of signals
An envelope detector can be used to
demodulate a previously modulated signal by removing all high frequency components of the signal. The capacitor and resistor form a low-pass filter to filter out the carrier frequency. Such a device is often used to demodulate AM radio signals because the envelope of the modulated signal is equivalent to the
baseband
In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable i ...
signal.
Audio
{{also, Noise gate#Trance gating
An envelope detector is sometimes referred to as an envelope follower in
music
Music is generally defined as the art of arranging sound to create some combination of form, harmony, melody, rhythm or otherwise expressive content. Exact definitions of music vary considerably around the world, though it is an aspe ...
al environments. It is still used to detect the
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
variations of an incoming signal to produce a control signal that resembles those variations. However, in this case the input signal is made up of audible frequencies.
Envelope detectors are often a component of other circuits, such as a
compressor
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
Compressors are similar to pumps: both increase the pressure on a fluid and both can tr ...
or an
auto-wah or envelope-followed filter. In these circuits, the envelope follower is part of what is known as the "
side chain
In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called the "main chain" or backbone. The side chain is a hydrocarbon branching element of a molecule that is attached to a ...
", a circuit which describes some characteristic of the input, in this case its volume.
Both
expanders and
compressors use the envelope's output voltage to control the gain of an amplifier. Auto-wah uses the voltage to control the
cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced ( attenuated or reflected) rather tha ...
of a filter. The
voltage-controlled filter of an
analog synthesizer
An analog (or analogue) synthesizer is a synthesizer that uses analog circuits and analog signals to generate sound electronically.
The earliest analog synthesizers in the 1920s and 1930s, such as the Trautonium, were built with a variety of ...
is a similar circuit.
Modern envelope followers can be implemented:
# directly as
electronic hardware
Electronic hardware consists of interconnected electronic components which perform analog or logic operations on received and locally stored information to produce as output or store resulting new information or to provide control for output a ...
,
# or as software using either a
digital signal processor
A digital signal processor (DSP) is a specialized microprocessor chip, with its architecture optimized for the operational needs of digital signal processing. DSPs are fabricated on MOS integrated circuit chips. They are widely used in audio s ...
(DSP) or
# on a general purpose CPU.
See also
*
Analytic signal
In mathematics and signal processing, an analytic signal is a complex-valued function that has no negative frequency components. The real and imaginary parts of an analytic signal are real-valued functions related to each other by the Hil ...
*
Attack-decay-sustain-release envelope
External links
Envelope detectorEnvelope and envelope recovery
Electronic music
Audio engineering
Communication circuits
Detectors
Demodulation