What Is A Descriptor ?
Descriptor is a characteristic feature or a word that is used to Describe and Identify a subject for information system.
In Other Words,
Descriptors are used to add, set and delete the attributes to an object. So it enable a software developer manage the attributes for an object by using these Descriptors. In general descriptors are defined as _get_ , _set_ and _delete_ .
Descriptors plays an important role in SI Tables (Service Information Tables). Each descriptor have its own Tag Value and also have its placement in SI Tables. Descriptors can also be used in other tables too.
What Is Descriptor Format ?
MPEG have defined format for Descriptors. This format is implemented by DVB or Digital Video Broadcasting as follows,
Descriptor_Tag –
It is used to identify the Descriptor. It is 8 bit field. Each Descriptor_Tag have its Value known as Descriptor Tag Value.
Descriptor_Length –
It is 8-bit filled. It defines descriptor’s data in bytes.
A Example of Bouquet Name Descriptor is show above.
What Are Descriptor Types ?
There are various descriptors used in SI for DVB. Lets have a look at some of these important descriptors,
(1) Bouquet Name Descriptor
This Descriptor Tag represented as bouquet_name_descriptor with Tag value of 0x47 . It contains 8 Bit Filed. It is used to define the name of bouquet. It is placed in BAT and SDT Tables.
(2) CA Identifier Descriptor
This Descriptor Tag represented as ca_identifier_descriptor with Tag value of 0x53 . This descriptor is found in BAT, SDT and EIT Tables.
It is used to define the Conditional Access System Type using CAS ID.
(3) Country availability Descriptor
This Descriptor Tag represented as Country_availability_Descriptor with Tag value of 0x49 . It is placed in Bouquet Association Table and Service Description Table.
This kind of descriptor is used to show availability of service in countries. So if there is only descriptor which is showing list of countries where service is available then it means that this service is available in the mentioned countries only while it is not available in other countries.
On the other hand if this descriptor is showing the list of countries where that particular service is not available. Then it means this service is available for all other countries except the mentioned countries in list.
In this type of descriptor flags are defined for a value. Where this flag value can be either 0 or 1. If the country flag is 0 then service for that country is not available. On the other hand if its value is set to 1 then service for that country is available.
Where country code is defined in 3 letters which are coded in binary form.
(4) Scrambling Descriptor
This Descriptor Tag represented as scrambling_descriptor with Tag value of 0x65 . It is placed in Program Mapping Table. This descriptor is used to define the scrambling mode for system. Where scrambling mode defines scrambling algorithms techniques.
(5) Linkage Descriptor
Its Descriptor Tag represented as Linkage_Descriptor with Tag value of 0x4A . It can be placed in NIT, BAT, EIT and SDT.
This type of descriptor is used to identify the additional information for a service. Just like if this descriptor is placed in Bouquet Association Table then it will provide some extra information about the bouquet.
Likewise if this descriptor is present in NIT then it will provide extra information about the Network.
There are many more descriptors which used in DVB like delivery system descriptors, Mosaic Descriptors, Transport Stream Descriptors and DSNG Descriptors etc.
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What are PSI SI Tables for DVB ?
reference : etsi
