Ericsson SPR1100 Operations Instructions download pdf (Page 293)

Automatic Loudness Control

The short term audio loudness correction is designed to correct spikes in loudness

such as those seen during interstitials / adverts. The response of this algorithm can

be controlled using the three available presets. The short term ALC algorithm will

never apply gain it will only attenuate the input, this is to avoid impairing intentionally

quiet sections of content, something that commonly occurs in movies. The algorithm

takes a measurement of loudness over a very short window and operates quickly to

correct and then remove correction once the problem has gone. If using only short

term ALC mode, for this to operate correctly then the loudness of the incoming audio

needs to be at approximately target level.

The long term audio loudness correction is designed to adjust the long term

loudness of a channel to the target level. This responds dynamically to the input and

the speed of response can be configured by the time period. This setting can be

used in a system to attempt to produce channels with equivalent loudness levels so

when a user is changing channels they do not perceive different loudness levels.

When transcoding, the measurement stage follows the decoding stage for the

incoming audio. All measurements and corrections are made to uncompressed

audio and are therefore independent of the choice of audio coding standards.

It is worth noting that although the measurement of audio loudness is specifically

defined by the standards mentioned in this document the correction is a result of the

interpretation of the specific algorithm. Therefore, two different implementations that

are both ITU-R BS.1770-2 compliant will not necessary produce the same corrective

result. It is therefore advised that if the corrected output is to be referenced to any

regional legislation that the measurement and any subsequent correction is

performed offline rather than on a real-time basis.

F.2 Operation

F.2.1 Loudness Units

LKFS

The ITU has defined a new unit for the measurement of audio loudness based upon

ITU-R BS.1770. This defines the designation LKFS, the unit is a perceptually

weighted measurement relative to full scale. A property of this unit is a relationship

to dBFS such that an increase in the level of a signal by 1 dB will cause an increase

in loudness by 1 LKFS.

The recommendation ITU-R BS.1771 defines the designation LU (Loudness Unit)

which has a one to one correspondence to LKFS.

2/1553-

FGC 101 1400 Uen B

F-5

1 2 ... 288 289 290 291 292 293 294 295 296 297 298 299 300

No comments

Ericsson SPR1100 Operations Instructions Page 293