It is recommended to provide one instance of ITvSoundChannel per sound output because this provides the most flexible control over sound channel selections and property observations.

Sound Channels

Sometimes one sound signal contains several audio streams that are not implicitly related to one another (for instance, a terrestrial transmission containing two separate languages). Since no one would ever want to hear all audio streams at the same time, it is very important that the selection of particular audio streams is possible. Depending on the type of sound signal, different audio streams can be present in one sound signal.

To simplify the selection of a particular audio stream three different abstract (logical) sound channels are defined. Each sound channel defines a preference that, together with the properties of the incoming signal, results in a particular audio stream selection:

�        Channel 1: mono preferred.
All signals are considered base audio signals, without additional modulations. In case of terrestrial transmitted sound, a mono carrier will be selected. In case of baseband (sound coming from an outside source), audio mono sound will be heard.

�        Channel 2: language A preferred (best possible if no dual language).
When selecting this channel, a user specifies not being interested in alternative languages. Still, the best possible sound quality is used. Consequently, Stereo and Digital quality sound will be selected if available. Only if no additional sound information is present in the signal, this channel has the same behaviour as channel 1.

�        Channel 3: language B preferred (best possible if no dual language).
When selecting this channel, a user specifies being interested in alternative languages. If no dual language is available, the behaviour the same as for channel 2.

Three types of sound signals can be identified: sound originating from an analogue terrestrial source, sound coming from a digital source, and sound coming from an external source (base band audio).

For analogue terrestrial sound, several transmission standards can be identified. Each of those transmission standards contains a base mono sound carrier (channel 1). Additionally, if the second carrier is present (depending on the transmission) extra information (indicating 2CS, BTSC, NICAM, or EIAJ) is being transmitted.

Table 1: Channel selection table for analogue sources

	Channel 1	Channel 2	Channel 3
Mono	Mono	Mono	Mono
Stereo (2CS)	Mono	Stereo	Stereo
Bilingual (2CS)	Mono (Language A)	Language A	Language B
NICAM Mono	Mono	NICAM Mono	NICAM Mono
NICAM Stereo	Mono	NICAM Stereo	NICAM Stereo
NICAM Bilingual	Mono	NICAM Language A	NICAM Language B
NICAM Data	Mono	Mono	Mono
NICAM unrelated Mono [1]	Mono	NICAM Mono	NICAM Mono
NICAM unrelated Stereo 1	Mono	NICAM Stereo	NICAM Stereo
NICAM unrelated Bilingual 1	Mono	NICAM Language A	NICAM Language B
NICAM unrelated Data 1	Mono	Mono	Mono
BTSC Stereo	Mono	Stereo	Stereo
BTSC Mono + SAP	Mono	Mono	SAP
BTSC Stereo + SAP	Mono	Stereo	SAP
EIAJ Stereo	Mono	Stereo	Stereo
EIAJ Bilingual	Mono	Language A	Language B
FM-Radio Mono	Mono	Mono	Mono
FM-Radio Stereo	Mono	Stereo	Stereo

 

Note that the table is the same for normal NICAM and unrelated NICAM systems. The only difference is that (as is stated in the remark) the detection of unrelated NICAM can result in a channel selection (thereby overruling the user setting). Unlike any other sound system, the unrelated NICAM system contains audio that is not belonging to the transmitted video. Therefore, the choice can be made to select the base audio channel (which still contains related audio) when unrelated NICAM is detected. Such channel change is then notified so one can decide explicitly to go over to unrelated audio. In this way, the least amount of audible effects is guaranteed.

 

Also for digital sources several transmission standards can be defined (e.g. PCM, MPEG, AC3, DTS). Digital sources distinguish themselves from analogue sources by the number of audio channels they can carry (they are not limited to stereo and mono transmissions). For those sources, it is useless to speak about a base mono sound carrier. Therefore, channel 1 selection will behave the same as the channel 2 selection. In the following table, all lines (except PCM Stereo) are valid for both AC3 and MPEG.

Table 2: Channel selection table for digital sources

	Channel 1	Channel 2	Channel 3
PCM Stereo	Stereo	Stereo	Stereo
1/0	1/0	1/0	1/0
2/0	Stereo	Stereo	Stereo
1+1	1/0 Language A	1/0 Language A	1/0 Language B
2+2	2/0 Language A	2/0 Language A	2/0 Language B
2/1	2/1	2/1	2/1
2/1 Karaoke [2]	2/0	2/0	2/0
2/2	2/2	2/2	2/2
2/2 Karaoke 1	2/0	2/0	2/0
3/0	3/0	3/0	3/0
3/0 Karaoke 1	3/0	3/0	3/0
3/1	3/1	3/1	3/1
3/1 Karaoke 1	3/0	3/0	3/0
3/2	3/2	3/2	3/2
3/2 Karaoke 1	3/0	3/0	3/0

 

Another type of sound signal is the one coming from an external source. Those base band audio signals discern themselves in the fact that they always carry only one sound channel. For similarity reasons it is chosen to downmix stereo base band signals to mono signal whenever possible if channel 1 is selected. For most external sources, a stereo sound signal will be present.

For external sources, it is useful to note that dual language transmissions are not possible, but it is possible to select left only (channel 1), left and right (channel2), and right only (channel 3). Considering this, the following channel selection table can be defined:

Table 3: Channel selection table for external sources

	Channel 1	Channel 2	Channel 3
Baseband� Mono	Mono	Mono	Mono
Baseband 2CS	Mono (Left)	Stereo (Left & Right)	Mono (Right)

 

Note that when the sound system is unknown, none of the logical channels has a valid sound signal. This is indicated by setting the number of front, rear, and LFE channels (see Sound Properties) to zero.

Sound Properties

For every sound signal, one can observe several properties. Using this interface, it is possible to investigate the properties of an audio signal. The following properties have been defined:

�       FrontChannels: The number of front channels reflects the number of independent audio channels (contained by the sound signal) intended for front speakers.

�       RearChannels: The number of rear channels reflects the number of independent audio channels (contained by the sound signal) intended for rear speakers.

�       LfeChannels: The number of audio channels intended for LFE (Low Frequency Effects) subwoofers.

�       Digital: Indicates whether the audio channel is analogue or digitally (including NICAM) encoded.

�       Related: Indicates whether the audio channel does not match the accompanied video.

�       SecondLanguage: Indicates whether the audio channel is the second (language) stream.

�       DolbyProLogic: Indicates whether a stereo signal contains Dolby Pro Logic sound.