DTV Audio: Encoder & Multiplexer Configuration

Encoder & Multiplexer Configuration

Broadcast DTV video encoders include options for multiple internal stereo AC-3 (Dolby Digital) encoders. This section describes configuration of these encoders and the associated multiplexer and PSIP system. These parameter are fully defined by the ATSC standard.

ATSC A/52A Digital Audio Compression (AC-3) Standard

If an external encoder is used instead, also perform the External Encoder Configuration.

Encoder - Dialnorm
First, you must insure that encoder input dialog levels are consistent. Then, unless otherwise instructed, set encoder dialnorm to the dialog level measured with a suitable LAeq meter at the encoder input. See Understanding Dialnorm.

Dolby LM100 Broadcast Loudness Meter

Encoder - DRC Profiles
We recommend applying minimal audio compression before the DTV audio system so the consumer can be given the option to select the amount of compression. (See Dynamic Range Control). If this is your approach, set the "Line Mode DRC Profile" and "RF Mode DRC Profile" to a "light" or "standard" film setting. If instead you apply heavy processing ahead of the DTV audio encoder, consider using a "light" or "none" film setting. While it seems like the RF mode should have a more aggressive setting, Dolby often recommends identical settings for both modes.

Encoder - RF Overmodulation Protection
Following Dolby's recommendation, set RF Overmodulation protection to "Disable".

Encoder - Bitstream Settings

Encoder - Data Rate
AC-3 provides a range of bit rates for each channel mode, allowing the broadcaster to trade bit economy for audio quality. Be aware that some early DTV receivers have proven to be incompatible with certain data rates. The data rate in the MPEG multiplexer must be set to this same value.

Encoder - Channel Mode
The naming convention indicates [ number of front channels / number of surround channels] .

Encoder - LFE Mode
LFE stands for Low Frequency Effects (subwoofer).

*Bitstream Mode & Audio Stream Descriptors
At first glance, these encoder and MPEG multiplexer settings look tempting to the broadcaster. For example, they seem to allow the user to choose from a variety of dialog channels in various languages to be mixed with 5.1 channels of music and effects. However, using these extended modes in DTV broadcasting would require dual Dolby decoders in every DTV and digital cable set-top box. This was not mandated by the FCC and there has been no hint of voluntary adoption by the cable and consumer electronics industries.

While there is no evidence that these parameters are used in receivers, setting them correctly is simple and provides insurance for future implementation. CM is "Complete Main" and VI is "Visually Impaired". The "asvcflag" setting shown for DVS associates it with the main English service.

PSIP Settings

PSIP - PID Assignment
Assign the main audio to the lowest PID since some receivers default to the first audio PID when tuning to a new channel.

PSIP - ISO Language Code
Some DTV receivers momentary display this ISO language identifier when the user selects between audio PIDs. This is NOT the A/52 "lancod" parameter, which is "reserved" (not used).

Table of ISO 639 Language Codes

Digital Cable
Digital cable systems use a proprietary "PSIP" system on a dedicated channel to inform set-top boxes of available channels and guide their tuning. These systems typically only allow selection of either the main audio or an optional Spanish track, with no provision for a separate Video Description channel. Inform your cable system of your audio configuration so that they can best program their system to match your services. If you want to offer simultaneous Spanish and Video Description to DTV viewers, cable viewers may be prevented from selecting Video Descriptions. Be very cautious about dynamically changing your multiplexer settings. This may disturb cable set-tops since the cable channel-mapping system is fixed and ignores DTV PSIP. See Digital Cable EPG.