Global Music Resource

Digital data Editing and Effects application can be done strictly by software applications and/or hardware devices. Some software and hardware will be designed specifically for one function, for instance just Compression. Some applications and/or devices will have multi-functions. DSP will be done fixed or floating point. The key will be in the strength of the internal circuitry and the processing capability of application algorithm within the hardware you purchase.

Dynamic Range is the range between the the highest amplitude and the lowest amplitude of a digital audio data signal.

Filtering

Most editing and effects are conducted by the application of a Filter. Filters remove, increase or shape frequencies from the digital audio signal.

The High Pass Filter (Treble Shelf Filter) cuts out frequencies below a preset level (cut-off frequency) or boosts frequencies above a preset level.

The Low Pass Filter (Bass Shelf Filter) cuts out frequencies above a preset level (cut-off frequency) or boosts frequencies below a preset level.

A Band Pass Filter (Mid Range Filter, Notch Filter) allows only a signal of a specific range (band) of frequencies to pass, and these frequencies can also either be cut or boosted. A sweepable band pass filter will allow one to reset the center frequency, which in turn will adjust the rest of the frequencies in the specified band that range both equally higher and lower from the center frequency. A Parametric Filter allows one to increase or decrease the specified range of frequencies that are encompassed in the band.

Anti-aliasing filtering is the application of a combination of pass band and stop band filters that do not allow incoming signal frequencies (say during analog to digital conversion) in access of 20.5 to 21.5 KHz so that Quantization Error will not occur.

Compression / Expansion

Compression is used to set the upper and lower frequencies of the dynamic range of the signal. Thus, compression will restrict the overall dynamic range of the signal so that it does not exceed a predetermined threshold level. High amplitudes are decreased to a maximum level while low amplitudes are increased to a minimum level. It is obtained by setting a maximum Threshold Level (measured in dB/decibels) and setting the compression ratio. Any signal that exceeds the Threshold is compressed (gain is reduced) in terms of the ratio. The ratio of compression to signal input reduces the dB level of the output signal by the ratio factor. Thus, in a 2 to 1 ratio, a signal that exceed the Threshold by 2 dB is compressed to only 1 dB gain increase. The higher the ratio, the more the gain reduction is applied. Conversely, on the low amplitude level, a 1 dB signal can be increased to 2 dB, thus increasing the volume of the low amplitude. The Attack (how quickly the Compressor will act on the signal) and Release (how quickly the Compression brings the gain back to the Threshold level) of the application of Compression must also be specified. These applications are measured in ms (milliseconds). The compression needs to be applied accurately and equally to both signals if it is an incoming stereo signal. In multi-band compression, different levels of compression can be applied to different frequencies of the signal.

Digital compression hardware have an ADC (Audio to Digital Converter) circuit that samples the incoming analog signal (or even an already existing digital signal). The variable voltage analog signal is sampled, the sample is a mathematical model of the amplitude of the analog signal represented by a bits / bytes sequence, and the compressor DSP application conducts a furhter manipulation of the amplitude model and the processed digital signal has the new amplitude model based on the parameters (Threshold Level, ratio, attack and decay) set by the user. A software compression application installed on a computer conducts compression in the same manner by emulating the function of an analog compressor and using the host processing capabilities of the CPU of the computer.

Expansion (Expander) is the opposite of compression, such that it increases the dynamic range (increases amplitude of the signal, amplifies, makes louder) of a signal if it drops below a threshold level. The attack and release of this affect can also be adjusted.

Limiter

A Limiter is used to establish a predetermined Threshold Level that the dynamic range of an incoming signal will not exceed without affecting the low end of the range.

Gating

Gating reduces or eliminates a signal if it decreases below a predetermined threshold level.

Noise Gate only allows certain frequencies through, which will reduce background noise or hiss and can actually be used to increase the dynamic range of a signal. The utilization of the noise gate requires specifying a threshold level (signal frequency) that will allow the signal to pass and then stop the signal once it falls below a certain frequency, thus keeping out any and all other signals.

Equalization

EQ or Equalization isolates frequencies, provide gain affects for how much the frequency is increased or reduced, and which range of frequencies are affected in order to balance the relationship between the highs and lows for a sound (frequency response). The “Q” in EQ is the measurement of the width of frequencies above and below an individual frequency. This width above and below the central frequency is measured in octaves. The actual application to the signal is done by shelf, peak and notch filters. By adjusting certain frequencies the actual perceptible analog sound quality is changed.

Equalization requires the application of a filter to modify the low, mid-range and high frequencies of an audio signal:

• High Pass Filter filters out specified low frequencies and only allows frequencies “higher” than the specified level (cut-off frequency) to pass into the mix.
• Low Pass Filter filters out specified high frequencies and only allows frequencies “lower” than the specified level (cut-off frequency) to pass into the mix.
• Band Pass Filter filters out frequencies above and below and specific range. A Band Reject Filter will perform the opposite, it will eliminate all frequencies within a specified range.

Graphic Equalization allows the amplitude (loudness) adjustment individually of several fixed frequency bands in a given audio signal. Each bandwidth can be adjusted independently of the other. The graph of the EQ Curve, which represents the intersection of amplitude of a signal (plotted on the y/vertical axis) and the frequency of the signal (the corresponding x/horizontal axis).

Parametric Equalization – allows macro control of bandwidth, frequency and amplitude (gain) for specific, overlapping bands of frequencies.

Normalization

Normalization insures that the amplitude of the cycles of the entire signal (or digital audio file) remain within the dynamic range.

Side Chain

A Side Chain is the ability of the dynamic range or effects processing (see below) hardware or software, or a mixing board, to apply a second (or more) effect(s) to a signal just prior to it is being routed / processed or immediately after.