The TyStream
Description of the Tivo TyStream and possible algorithms to extract the multiplexed video and audio stream in such way that it later on can be multiplexed without loosing sync of audio and video.
Definitions:
f/s: Frames per second, number of frames per second in a MPEG video stream.
Abbreviations:
lsb least significant byte first e.g. 16 bit integer is stored as (hex) XX YY where YY most significant byte and XX the least (e.g. 256 is stored like this in ff 00 and 1024 would be stored like this 00 04).
Unless otherwise noted all other values are stored with the most significant bit (note bit) first i.e. as you normaly would write numbers no matter binary, hex or ten based.
Description of the TyStream
A TyStream is built up of integer number of packets. Each Ty packet(also called chunk)is 128kBytes in size, and contains a integer number of ty_record_headers containing information about the multiplexed MPEG video, MPEG audio|AC3 audio and Data payload in the ty_packet_payload. The ty_stream uses a proprietary multiplexing schema of publicly unknown format. (Hence this document trying to describe it).
ty_stream(){
for(i=0; i < ty_stream_size/ty_packet_size; i++){
ty_packet()
}
}
ty_packet(){
ty_packet_header()
for(j=0; j < number_of_ty_records; j++){
ty_record_header()
}
ty_packet_payload()
stuffing_bytes()
}
ty_packet_header(){
if(marker_bit_1 == 1) {
number_of_ty_records 16 bits lsb
seq_record_nr [0-7] 8 bits
marker_bit_1 1 bit
seq_record_nr [8-14] 7 bits
} else {
number_of_ty_records 8 bits
seq_record_nr 8 bits
reserved 8 bit
maker_bit_2 1 bit
reserverd 7 bits
}
}
ty_record_header() {
if(marker_bit_2 != 1) {
marker_bit_2 1 bit
ty_record_size 19 bits
ty_record_type 12 bits
tivo_stream_buffer_id 12 bits
tivo_stream_buffer_offset 20 bits
ty_time_stamp 64 bits
} else {
marker_bit_2 1 bit
reserved 3 bits
extended_data 16 bits
ty_record_type 12 bits
unknown_reserved 32 bits
ty_time_stamp 64 bits
}
}
ty_payload() {
ty_record_data sum_of_all_record_size* bytes
}
stuffing_bytes() {
while(ty_packet_size < 131072) {
suffing_byte 8 bits
}
}
ty_stream
The TyStream (file) holding all audio, video and data for a Tivo recording.
ty_packet_size
A tystream has N ty_packets each 131072 bytes in size. Hence ty_packet_size == 131072. (131072 == 128kB)
ty_stream_size
The file size of the ty_stream in bytes.
ty_packet
An individual 131072 bytes sizes data packet holding video, audio and data multiplexed together.
ty_packet_header
Depending if the ty_stream is recorded by Tivo software revisions 1.3 or 2.0 (and higher) the header of the packet is slightly different. If marker_bit_1 is set then it's a 2.0 or higher recording.
number_of_ty_records
Holds the total number of ty_records present in the ty_packet.
seq_record_nr
Holds the record number of ty_record who has a payload that holds the start of a MPEG Video Sequence header. If no such ty_record is present in the ty_packet all bytes are set to 1.
ty_record_header
There are two types of record headers, one that has an additional payload in the present in the ty_payload and one that has the payload embedded in the record header it self (extended_data). If marker_bit_2 is set then the payload is embedded.
marker_bit_2
If set the the payload of the record is embedded in the header in the extended_data field.
ty_record_size
The size of the payload that the ty_record_header represents.
ty_record_type
Each ty_record_header has an associated ty_payload, the ty_record_type determines what type of data that is present in the ty_payload.
ty_record_types listed in hex:
0x3c0:
A MPEG PES header start code for a MPEG stream of MPEG 1 AUDIO type will start in the ty_record_data. NOTE: In a Dtivo TyStream this will be a MPEG1 PES header not a MPEG2 PES header.
0X9c0:
A MPEG PES header start code for a MPEG stream of AC3 audio type will start in the ty_record_data.
0X4c0:
A MPEG 1 AUDIO Layer II header will start in the ty_record_data. (This record_type is up until today only reported in SA Tivo).
0x2c0:
A generic AUDIO payload is present in the ty_record_data. This ty_record can only follow after a 0x3c0, 0x4c0 or 0x9c0 ty_record. It holds the reminder of the payload from the previous ty_record.
0x6e0:
A MPEG PES header start code sequence for a MPEG stream of MPEG 2 VIDEO type will start in the ty_record_data. (This record_type is up until today only reported in Series 1 Tivo).
0x7e0:
A MEPG 2 VIDEO Sequence start code will start in the ty_record_data.
0xce0:
A MEPG 2 VIDEO GOP start code will start in the ty_record_data.
0x8e0:
A MEPG 2 VIDEO picture start code will start in the ty_record_data. The MPEG picture data will be a I-Frame.
0xae0:
A MEPG 2 VIDEO picture start code will start in the ty_record_data. The MPEG picture data will be a P-Frame.
0xbe0:
A MEPG 2 VIDEO picture start code will start in the ty_record_data. The MPEG picture data will be a B-Frame.
0x2e0:
A generic MEPG 2 VIDEO payload is present in the ty_record_data. This ty_record can only* follow after a 0x8e0, 0xae0 or 0xbe0 ty_record. It holds the reminder of the payload from the previous ty_record.
*This is not yet confirmed!
0xe01:
Closed Captions (CC), the payload is present in the extended_data field.
0xe02:
Extended Data Service (XDS), the payload is present in the extended_data field.
0xe03:
Tivo Ipreview ty_record. This is the ty_record responsible for the "thumbs up" during previews.
0xe04:
UK ??TeleText?? data - Not yet confirmed
0xe05:
UK ??TeleText?? data - Not yet confirmed
0xe06:
This is a Tivo data, it's used to carry things like e.g. the previews that you sometimes can get in your top Tivo menu.
0x000:
This is a packet used by Tivo when Tivo didn't manage to encode on a SA / read the Transport Stream on a DTivo. It's a good indicator that we have serious damage in the stream. Tivo is simply dumping the data it manged to read into this record.
extended_data
ty_records having having maker_bit_2 set has it's 2 byte "ty_record_data" present in this field.
tivo_stream_buffer_id
tivo_stream_buffer_id used internally in the Tivo*
tivo_stream_buffer_offset
Offset in the specified tivo_stream_buffer*
ty_time_stamp
Timestamp scaled by 1e9 most propably used internally during e.g. fast forward of the video stream*
*Not yet confirmed
ty_payload
The ty_payload holds all the ty_record payloads multiplexed together.
ty_record_data
A ty_record_data is the corresponding payload to a ty_record. The ty_record_data is a segment of the ty_payload of the size specified in the ty_record_size in the ty_record_header. The offset to from the start of the ty_record_data segment relative to the start of the ty_packet is 4 + 16 * nr_of_ty_records + ty_record_size (ty_record_N -1).
stuffing_bytes
If the ty_packet is short of data i.e. the ty_payload ends before 131072 bytes, the reminder of the packet will be filled with random data.
Description of the Video, Audio and Data present in the a ty_packet
The ty_packtet is a proprietary multiplexed MPEG-2 video/audio stream that can be best described as a mix between a program and a transport stream with essential parts of the stream lacking.
MPEG Video present in the TyStream:
Even if the video(audio) stream is resembling a transport stream it's still very far from actually being one. There is no transport or program stream headers present in the TyStream. The higest level of MPEG packet header present is the PES packet header.
As in a MPEG transport stream the size field present in the PES header measures zero. Hence there is way to measure the size of a individual PES video packet. The size used by Tivo is instead the size field present in the ty_record header.
Further more the PES video packet present in the TyStream only has the PTS time stamp present and there is no DTS time stamp present which is necessary for proper decoding.
The PES packeting and sequence/gop "packeting" of the Tivo stream is also slightly different between the different Tivo software revisions, Tivo series, and Tivo Type
SA Tivo Series 1 version 1.3:
Version 1.3 has only one PES packet per sequence/gop header. In other words you have the following sequence PES header, sequence header, gop header, video data (I, P and B frames), PES header and so on. All GOP's are open GOP's even the start GOP. The broken link is not enabled in the start GOP even if it should in order to comply with the MPEG-2 video standard.
SA Tivo Series 1 version 2.x and higher:
Version 2.x (and higher) has one PES packet per I and B frame. PES packets holding B frames can also include P frames, sequence headers and gop headers. A video sequence can there fore look like this: PES header, B-Frame, sequence header, GOP header, PES header, I-Frame, PES header, B-Frame, PES header, B-Frame, P-Frame, and so on. All GOP's are open GOP's even the start GOP. The broken link is not enabled in the start GOP even if it should in order to comply with the MPEG-2 video standard.
DirecTV Tivo Series 1 version 2.x and higher:
DTivo 2.x (and higher) has one PES packet per I, B and P frame. PES packets holding B frames can also include sequence headers and gop headers. A video sequence can there fore look like this: PES header, B-Frame, sequence header, GOP header, PES header, I-Frame, PES header, B-Frame, PES header, B-Frame, PES header P-Frame, and so on. All GOP's are open GOP's even the start GOP. The broken link is not enabled in the start GOP even if it should in order to comply with the MPEG-2 video standard.
SA Tivo Series 2 version 3.x and higher:
Series 2 has one PES packet per Sequence header, B and P frame. PES packets holding Sequence headers are also including GOP headers and I-Frames. All GOP's are open GOP's with the exception of the first GOP which is closed. However the even if the first GOP is closed it's marked as open in the GOP header.
DirecTV Tivo Series 2 version 3.x and higher:
UPDATE: Have S2 Dtivo streams now and it's like a mix between S1 Dtivo and S2 SA :) tydemux is demuxing it properly if it has AC3 sound. Need to update this section.
At present time I have not yet got hold of a streams from a DirecTV Tivo series two. However since Tivo is using the DirecTV DDS MPEG stream and not encoding the stream it self. It should not be different from a Series 1 DirecTV Tivo, but I can't confirm until I got hold of streams from a Series 2 DirecTV Tivo.
All:
Noticeable is also the total lack of video sequence end codes, there should be one at least at the end of the recording.
Reference:
ISO/IEC 13818-1 MPEG 2 System Specification
ISO/IEC 13818-2 MPEG 2 Video Specification
ISO/IEC 11172-3 MPEG 1 Audio Specification
EIA-608 Closed Caption/Extended Data Service Specification
ATCS A/52A AC-3 Specification