The SDC contains further information about the multiplex - this includes a label of up to 16 characters and alternate frequency information. The SDCís frame is broadcast every 1.2 Sec. The SDC contains: multiplex description, label, conditional access, frequency information, frequency schedule information, application information, announcement support and switching, coverage region identification, time and date information, audio information, FAC copy information, and linkage data.
SDC describes the audio coder used, and the parameters associated with that coder. It also provides information about the sampling rate and bit rate used. The SDC gives information on how to decode the MSC, how to find alternative sources of the same data, and gives attributes to the services within the multiplex.
The data capacity of the SDC varies with the spectrum occupancy of the multiplex and other parameters. As well as conveying this data, the fact that the SDC is inserted periodically into the waveform is exploited to enable seamless switching between alternate frequencies.
An SDC block is the SDC data contained in one transmission super frame. The SDC is treated as a single data channel. The total amount of data to be sent may require more than a single SDC block to send. An AFS index is therefore provided to permit a receiver to know when the next occurrence of the current SDC block will be transmitted, and so allow for alternative frequency checking and switching (AFS).
The AFS index is an unsigned binary number in the range 0 to 15 that indicates the number of transmission super frames which separate this SDC block from the next with identical content when the identity field in the FAC is set to 00.
The SDC may use either 16-QAM or 4-QAM mapping. 16-QAM provides greater capacity whereas 4-QAM provides a more robust error performance. In each case, a fixed code rate is applied. The constellation with respect to the MSC parameters provide more robustness for the SDC than for the MSC.
alternative frequency checking and switching (AFS)
alternative frequency checking and switching (AFS)
The content of the SDC is important for the operation of alternative frequency checking and switching (AFS). For AFS to function, the receiver must know what the content of the SDC is in advance so that a correlation may be performed. For this purpose, the AFS index is provided in the SDC and the FAC validates the index by use of the Identity field.
On transmissions with alternative frequencies, the assignment of data entities to SDC blocks should be carefully designed in order that the content of the SDC can be as static as possible thereby permitting use of the AFS function. In this case it is recommended that the AFS index is chosen such that all required information can be sent in one cycle of SDC blocks. If the content is completely static then the Identity field in the FAC will indicate the sequence 00, 01, 10, etc. which indicates that the AFS function can be performed at every position, provided the receiver has stored the data for all the SDC blocks.
When the time or announcement support and switching is included in the SDC, and alternative frequencies are signalled, then a semi-dynamic use of the SDC is recommended. In this case one or more SDC blocks in the cycle defined by the AFS index are signalled to be invalid by use of the FAC Identity field thereby allowing the content of those blocks to be changed continuously, whilst other SDC blocks are always signalled as valid by use of the FAC Identity field thereby allowing the AFS function to be performed. A change of the AFS index is only allowed at reconfiguration.
Reconfiguration of the DRM multiplex is signalled in advance in order to permit receivers to make the best decisions about how to handle the changes. There are two types of reconfiguration: a service reconfiguration, which concerns the reallocation of the data capacity between the services of the MSC; and a channel reconfiguration, which concerns changes to the overall capacity of the MSC.
Both types of reconfiguration are signalled by setting the FAC reconfiguration index to a non-zero value. The index then counts down on each subsequent transmission super frame. The final transmission super frame corresponding to the current configuration shall be that in which the reconfiguration index = 1. The new configuration takes effect for the next transmission super frame and in which the reconfiguration index = 0. Ideally the reconfiguration should be signalled as far in advance as possible (i.e. the reconfiguration index should first take the value 7) in order to provide the greatest chance that the receiver will receive all the information necessary for the next configuration.
A service reconfiguration is one in which the data capacity of the MSC is reallocated between services. This happens when the number of services in the multiplex is changed or the size of data streams is changed. When a new service is introduced, and the overall capacity of the MSC is not changed, then the receiver shall follow the currently selected service through the reconfiguration. To facilitate this, the Service Identity and Short Id of all continuing services shall remain the same. The new service shall use a Short Id that is not used in the current configuration.
A channel reconfiguration is one in which the following FAC channel parameters are altered: spectrum occupancy, interleaver depth and MSC mode; and when the robustness mode is changed. In this case the receiver will be unable to follow the currently selected service without disruption to the audio output.
If the transmission is to be discontinued on the tuned frequency, then a reconfiguration shall be signalled with data entity type 10 taking a special value.
Main Service Channel
MSC contains the data for all the services within the DRM multiplex. The multiplex may contain between one and four services, and each service may be either audio or data.
DRM can also transmit an Electronic Programme Guide. This data is sent as an XML file as part of the MSC. The broadcast time and name of the programme is transmitted and for each programme there is an associated genre. There are hundreds of pre-defined genre types, this will make it possible to look for specific types of radio programme within the EPG.