Audio Mixer Technical Details

decibel relative to Full Scale (dBFS)

Audio is measured in decibel (dB), which is a logarithmic scale which closely resembles how our ears and brain perceive audio volume.

dB is a relative measurement. We could put the value 0 dB anywhere on the meter and be correct. In digital audio it is the convention to use 0 dBFS (the FS suffix is to indicate this convention) as the maximum volume that the sound card, audio interface, DA/AD-converter can handle. Lower volume levels are shown with negative dBFS values.

OBS internally uses floating point calculations for audio processing, so it does have the ability to process audio that is louder than 0 dBFS. However in the end when OBS records or streams the video, the audio will need be below 0 dBFS or the viewer will hear a nasty distortion called clipping (described at this start of this guide).

The Peak Programme Meter (PPM)

The PPM is the main visual feature on the OBS meter, lighting up as part of the overall volume meter.
The length of the bar indicates the peak volume level of what the viewer will hear when playing your video or stream. You can change the volume by moving the fader which is directly below.

The meter is split into three different coloured sections. At -20 dBFS we have the Alignment Level (AL) and at -9 dBFS we have the Permitted Maximum Level (PML).

Traditionally a sinus tone is played through the whole system and gain is configured for each piece of equipment to show the tone to be at exactly the -20 dBFS Alignment Level (AL). This way the level on your mixing desk will match that of OBS. This is important if you have a lot of audio equipment in a chain. The value for the alignment level was chosen to be near the average audio level for speech.

The Permitted Maximum Level (PML) is the level where if you go above this value there is a small but potential chance that the sound will get clipped before it reaches the viewer. -9 dBFS was chosen because:

• With most PPMs, including the one in OBS, the audio may have a peak level 3 dB higher than what is read from the meter.
• When reading PPMs it is difficult to see the actual peak, so another 3 dB margin is added. This problem is eliminated in OBS due to the peak-hold feature which makes reading accurate.
• Alignment errors, with multiple pieces of equipment in the chain an extra 3 dB margin was added for difference is levels.

OBS currently implements a "Sample peak program meter (SPPM)", in the future it would be preferred to replace it with a proper "4x Over-sampling peak programme meter", which would make it more accurate for measuring maximum peaks.

Peak and Hold

There is a small line that moves right with the PPM meter, but then stays there when the PPM moves left again. It will stay there for 20 seconds before returning back to where the PPM meter currently is. This allows you to easily check what the maximum level was after you accidentally made a loud noise.

VU-meter

A second small line on the meter, black and inside the bar of the PPM meter, is a VU-meter. This meter was traditionally used to determine loudness, because it was cheap to manufacture, and in OBS easy to implement.

It measure the root-mean-square, integrated over a period of 300 ms. Due to the calculation it shows more closely the sound pressure levels than does a peak-meter.

This meter is less useful, but it kept some structure of the code intact so we can replace it with a proper loudness meter based on ITU-R BS.1770-2.

Input Level

The input level are the small square indicator at the far left of the meter. This is the best place to see if the audio is too loud for the audio interface that captures your microphone.

The colours have the following meters:

• dark green: the input level is less than -50 dBFS
• light green: the input level is between -50 and -20 dBFS
• yellow: the input level is between -20 and -9 dBFS
• red: the input level is between -9 dBFS and -0.5 dBFS
• white: the input level is larger than -0.5 dBFS

If the indicator is missing it means there is no audio streaming toward OBS. This may be simply due to no audio being available yet, waiting for the user to start playback of an audio file. Or it may indicate a problem like the audio interface having been disconnected.

The input level meter is before the volume fader, but it is behind any filters that are in use by a source. To correctly determine input level you will need to disable the filters for this source.