Difference between revisions of "Threshold"

Latest revision as of 15:41, 27 February 2019

Overview

The term "threshold" as applied to audio refers to a specific volume level where a process begins to take effect. In some cases the threshold is variable by the user and in other cases it is at a "fixed' level.

In digital circuitry, the term can apply to the voltage level reached by an input that results in the output of the device in question changing states. This is an important concept in understanding how jitter can result from variations in the waveform input to clock recovery circuitry.

Basics

In most cases, a threshold is a specific voltage level, largely because most signals under consideration exist as a voltage waveform. The threshold voltage level is compared to the source voltage level, which may be a waveform in cases where the instantaneous level is important or a processed signal when a more generalized or “average” level is important. When the input voltage level exceeds the threshold voltage level, a signal is generated.

Processing of analog signals for level control typically involves rectifying the AC waveform into a DC waveform and filtering this waveform to create a varying DC level. This level typically represents the average or R.M.S. voltage level of the original audio signal, but can also represent the peak level in applications such as audio limiting.

Digital Clock recovery

Ideal digital electronic signals have infinitely short time intervals for the transition between a “0” and a “1” and vice-versa. This will be referred to as a change of “state” and corresponds to the signal voltage changing between very close to ground or 0 volts and very close to the positive power supply voltage for the circuitry (+5VDC in TTL circuitry). For binary applications, the 0 Volt portion represents a binary “zero” and the 5V portion represents a binary “one.”

Real world signals do not change states in zero time, and this implies that some method must be used to decide when the input signal changes state.

This is accomplished by having a voltage threshold level at the input of the receiving device. Ideally this level would be approximately half of the total voltage range available, but for reasons that are beyond the scope of this discussion, it is typically closer to 25-35% of the total voltage range. As the voltage level of the input waveform slopes upward (or downward) it crosses this threshold, and at this point in time the output of the receiving device changes state.

Anything that changes the shape of the waveform can affect the relative timing of when the threshold is crossed, which is why the primary cause of jitter in signals transmitted between devices is waveform distortion. The single most important source of waveform distortion is reflections in the transmission chain caused by impedance miss-matches. Use of impedance matched cable and connectors as well as proper termination helps to greatly minimize impedance miss-matches in the chain. Interference caused by electromagnetic or electrostatic pickup on the cable or ground loops between equipment can also result in distortion of the waveform.

Analog Dynamic Range Control

In order to "fit" wide dynamic range audio signals into limited dynamic range recordings; some form of volume level control is necessary.

There are two types of volume control:

Manual- Typically a Volume or Gain control on the recording device or a device which is feeding the inputs of the recording device.
Automatic- Can be any one or a combination of the following: Compressor, limiter, expander/noise gate, analog soft limiting (or "analog saturation"), digital soft saturation.

In the case of compressor, limiter, or expander/noise gate the threshold is typically user-controllable in the form of a Threshold adjustment or a combination of input and output level controls. Because saturation modes are basically peak level controls; their thresholds are generally fixed near peak volume level.

The basic premise is that for signals with a volume level lower than the threshold the signal is unchanged. As the volume level increases and exceeds the threshold, the process begins. In some cases, the process is applied in a linear manner where every increase of the input level by one dB results in the same amount of change to the output signal, and in other cases the process has a more "progressive" non-linear effect where the effect is minimal just above the threshold and increases as the input level increase. The saturation modes of Lavry AD converters are an example of non-linear processes. Another example is the "soft-knee" compressor or limiter.

The one exception in the examples provided is in the case of an expander/noise gate; in which case the signal is unchanged above the threshold and processed below the threshold.

Lavry Products

LavryGold AD122-96 MX for more information click here
LavryGold AD122-96 MkIII for more information click here
LavryBlue MAD-824 for more information click here
LavryBlack AD11 for more information click here

@@ Line 1: / Line 1: @@
 ==Overview==
 The term <nowiki>"threshold"</nowiki> as applied to audio refers to a specific volume level where a process begins to take effect. In some cases the threshold is variable by the user and in other cases it is at a "fixed' level.
+In digital circuitry, the term can apply to the voltage level reached by an input that results in the output of the device in question changing states. This is an important concept in understanding how [[jitter]] can result from variations in the [[waveform]] input to clock recovery circuitry.
 ==Basics==
+In most cases, a threshold is a specific [[voltage]] level, largely because most signals under consideration exist as a voltage [[waveform]]. The threshold voltage level is compared to the source voltage level, which may be a waveform in cases where the instantaneous level is important or a processed signal when a more generalized or “average” level is important. When the input voltage level exceeds the threshold voltage level, a signal is generated.
+Processing of analog signals for level control typically involves rectifying the [[AC]] waveform into a [[DC]] waveform and filtering this waveform to create a varying DC level. This level typically represents the average or [[R.M.S.]] voltage level of the original audio signal, but can also represent the peak level in applications such as audio limiting.
+==Digital Clock recovery==
+Ideal digital electronic signals have infinitely short time intervals for the transition between a “0” and a “1” and vice-versa. This will be referred to as a change of “state” and corresponds to the signal voltage changing between very close to [[ground]] or 0 volts and very close to the positive power supply voltage for the circuitry (+5VDC in TTL circuitry). For binary applications, the 0 Volt portion represents a binary “zero” and the 5V portion represents a binary “one.”
+Real world signals do not change states in zero time, and this implies that some method must be used to decide when the input signal changes state.
+This is accomplished by having a voltage threshold level at the input of the receiving device. Ideally this level would be approximately half of the total voltage range available, but for reasons that are beyond the scope of this discussion, it is typically closer to 25-35% of the total voltage range. As the voltage level of the input waveform slopes upward (or downward) it crosses this threshold, and at this point in time the output of the receiving device changes state.
+Anything that changes the shape of the waveform can affect the relative timing of when the threshold is crossed, which is why the primary cause of [[jitter]] in signals transmitted between devices is waveform distortion. The single most important source of waveform distortion is reflections in the transmission chain caused by [[impedance]] miss-matches. Use of impedance matched cable and connectors as well as proper [[termination]] helps to greatly minimize impedance miss-matches in the chain. Interference caused by electromagnetic or electrostatic pickup on the cable or [[ground loop]]s between equipment can also result in distortion of the waveform.
+==Analog Dynamic Range Control==
 In order to "fit" wide [[dynamic range]] audio signals into limited dynamic range recordings; some form of volume level control is necessary.
 There are two types of volume control:
-#Manual- typically a "volume" or "Gain" control on the recording device or a device which is feeding the inputs of the recording device.
+#Manual- Typically a Volume or Gain control on the recording device or a device which is feeding the inputs of the recording device.
 #Automatic- Can be any one or a combination of the following: Compressor, limiter, expander/noise gate, analog soft limiting (or "analog saturation"), digital [[soft saturation]].
-In the case of compressor, limiter, or expander/noise gate the threshold is typically user-controllable in the form of a "threshold" adjustment or a combination of input and output level controls. Because saturation modes are basically "peak level" controls; their thresholds are "fixed" near peak volume level.
+In the case of compressor, limiter, or expander/noise gate the threshold is typically user-controllable in the form of a Threshold adjustment or a combination of input and output level controls. Because saturation modes are basically ''peak level'' controls; their thresholds are generally ''fixed'' near peak volume level.
 The basic premise is that for signals with a volume level ''lower than the threshold'' the signal is unchanged. As the volume level increases and exceeds the threshold, the process begins. In some cases, the process is applied in a [[linear]] manner where every increase of the input level by one dB results in the same amount of change to the output signal, and in other cases the process has a more "progressive" [[non-linear]] effect where the effect is minimal just above the threshold and increases as the input level increase. The [[saturation]] modes of Lavry AD converters are an example of non-linear processes. Another example is the "soft-knee" compressor or limiter.
@@ Line 14: / Line 30: @@
 *The one exception in the examples provided is in the case of an expander/noise gate; in which case the signal is unchanged ''above the threshold'' and processed ''below the threshold''.
 ==Lavry Products==
+*LavryGold AD122-96 MX [http://www.lavryengineering.com/products/pro-audio/ad122-96-mx.html for more information click here]
 *LavryGold AD122-96 MkIII [http://www.lavryengineering.com/products/pro-audio/ad122-96-mkiii.html for more information click here]
 *LavryBlue MAD-824 [http://www.lavryengineering.com/products/pro-audio/lavryblue-m-ad-824.html for more information click here]
 *LavryBlack AD11 [http://www.lavryengineering.com/products/pro-audio/ad11.html for more information click here]
-==Overview==
-The term "<nowiki>dynamic range"</nowiki> is used to define the difference between the highest and lowest level an audio signal can have in a circuit or device. The lowest level is typically limited by noise in the system. At some point the signal level will be low enough for the signal to be obscured by the noise. At the other end of the scale, the loudest signal level is typically defined by a certain level of [[distortion]]. In [[analog]] audio exactly what level of distortion is used varies with the application; and in the case of [[digital audio]], corresponds to [[full scale]] or "digital clipping" level where all [[bits]] in the digital [[word]] which represent audio are "1."
-==Basics==
-Dynamic range in audio is quite often specified as the "signal-to-noise ratio" ([[SNR]]). It is typically specified in decibels ([[dB]]) which is a ''relative'' measurement- the ratio of the power of the signal to the power of the noise. The "signal" means the highest level signal the circuitry can handle without distortion and the "noise" is typically present in the circuitry whether or not the signal is present. The highest level the circuitry can handle is usually the point where a specific level of distortion is used to signify the ''onset of distortion.'' This means that the signal level will only need to be increase slightly for the distortion level to increase significantly. If the onset of distortion is more gradual, this level represents when the distortion becomes audible.
-Exactly what is considered to be ''audible'' distortion can vary with the type of device being measured. For example; in loudspeaker testing levels as high as 3-5 % distortion are not unusual compared to one-half a percent or less for high quality audio amplifiers. As a result; the distortion level used to determine the peak level as well as the type of distortion is usually also specified as part of the SNR or dynamic range specification.
-The other factor is the rate of the on-set of distortion. In digital audio or amplifier circuits with op-amps; the highest signal level is quite often just below "[[clipping]] level" and the signal below that is very low in distortion.  Due to the way in which none of the original information is retained above the clipping level, the onset of distortion is quite rapid beyond this level. This means a very low level of distortion can be used to signify the peak level because even a small increase in distortion will indicate being very close to the gross distortion level. In the case of a speaker or analog tape recording; the onset of distortion is much more gradual. This makes defining "the point" at which the distortion increases to an unacceptable level more arbitrary and subjective.
 [[Category:Terminology]]