SV8 Specification¶
Note: All fields, unless explicitly specified otherwise are read and written in Big-Endian order.
Status of this document¶
This document is a work in progress, but part of the document will not be changed.
Here is how the status is indicated for each part:
| Status | Meaning |
|---|---|
| [final] | This part is final and will not be changed |
| [beta] | This part will not be changed unless necessary |
| [alpha] | This part is still discussed and will probably change |
| nothing specified | This part is here for discussion, and not part of the specification |
File magic number¶
[final] Magic number is on 32bits and is equal to ‘MPCK’ or 0x4D50434B
File extension¶
[final] The prefered file extension for musepack files is .mpc
Packet formatting¶
[final] All packets are formatted using Key / Size / Payload. Keys are 16 bits long. It’s equivalent to the packet ID or type. Size is a variable-size field:
bits, big-endian
0xxx xxxx - value 0 to 2^7-1
1xxx xxxx 0xxx xxxx - value 0 to 2^14-1
1xxx xxxx 1xxx xxxx 0xxx xxxx - value 0 to 2^21-1
1xxx xxxx 1xxx xxxx 1xxx xxxx 0xxx xxxx - value 0 to 2^28-1
...
Size defines the packet length in bytes, including the Key and Size fields. So the minimum length of a block is 3 bytes. The payload is the actual packet data. Its size can be null. All unused bits in a packet MUST be null.
| Field | Size (bits) | Value |
|---|---|---|
| Key | 16 | “EX” |
| Size | n*8; 0 < n < 10 | 0x1A |
| Payload | Size * 8 | “example” |
Summary of reserved packet keys¶
Allowed chars in key are [A-Z] (65 <= value <= 90), so 676 keys are valid out of 65536 possible.
| Packet Name | Key | Mandatory | Status |
|---|---|---|---|
| Stream Header | SH | yes | [final] |
| Replaygain | RG | yes | [final] |
| Encoder Info | EI | no | [final] |
| Seek Table Offset | SO | no | [final] |
| Audio Packet | AP | yes | [final] |
| Seek Table | ST | no | [final] |
| Chapter-Tag | CT | no | [beta] |
| Stream End | SE | yes | [final] |
Stream Header Packet¶
[final] This packet key is “SH”. It contains the information needed to decode the stream. This block is mandatory and must be written before the first audio packet.
| Field | Size (bits) | Value | Comment |
|---|---|---|---|
| CRC | 32 | CRC 32 of the block (this field excluded). 0 = invalid | |
| Stream version | 8 | 8 | Bitstream version |
| Sample count | n*8; 0<n<10 | Number of samples in the stream. 0 = unknown | |
| Beginning silence | n*8; 0<n<10 | Number of samples to skip at the beginning of the stream | |
| Sample frequency | 3 | 0..7 | See table below |
| Max used bands | 5 | 1..32 | Maximum number of bands used in the file |
| Channel count | 4 | 1..16 | Number of channels in the stream |
| MS used | 1 | True if Mid Side Stereo is enabled | |
| Audio block frames | 3 | 0..7 | Number of frames per audio packet (4^value=(1..16384)) |
Do we need to specify the channel position ? There seems to be no standard for positions / order of the channels.
The SV8 stream allows to start decoding only on the first packet frame, so cutting the stream requires cutting on packet boundaries. Beginning silence and sample count fields provide more precise cutting possibilities.
Frequency table
| Value | Frequency (Hz) |
|---|---|
| 0 | 44100 |
| 1 | 48000 |
| 2 | 37800 |
| 3 | 32000 |
The CRC used is this one : http://www.w3.org/TR/PNG/#D-CRCAppendixhttp://www.w3.org/TR/PNG/#D-CRCAppendix.
Replaygain Packet¶
[final] This packet key is “RG”. It contains the necessary data needed to apply replaygain on the current stream. This packet is mandatory and must be written before the first audio packet.
| Field | Size (bits) | Value | Comment |
|---|---|---|---|
| ReplayGain version | 8 | 1 | The replay gain version |
| Title gain | 16 | The loudness calculated for the title, and not the gain that the player must apply | |
| Title peak | 16 | ||
| Album gain | 16 | The loudness calculated for the album | |
| Album peak | 16 |
The replay gain values are stored in dB in Q8.8 format. The 0 value means that this field has not been computed (no gain must be applied in this case. exemples :
- Replay gain finds that this title has a loudness of 78.56 dB. It will be encoded as 78.56 * 256 ~ 20111 = 0x4E8F
- For 16-bit output (range [-32767 32768]), the max is 68813 (out of range). It will be encoded as 20 * log10(68813) * 256 ~ 24769 = 0x60C1
- For float output (range [-1 1]), the max is 0.96. It will be encoded as 20 * log10(0.96 * 2^15^) * 256 ~ 23029 = 0x59F5 (for peak values it is suggested to round to nearest higher integer)
Encoder Info Packet¶
[final] This packet key is “EI”.
| Field | Size (bits) | Value | Comment |
|---|---|---|---|
| Profile | 7 | 0..15.875 | quality in 4.3 format |
| PNS tool | 1 | True if enabled | |
| Major | 8 | 1 | Major version |
| Minor | 8 | 17 | Minor version, even numbers for stable version, odd when unstable |
| Build | 8 | 3 | Build |
Seek Table Offset Packet¶
[final] This packet key is “SO”. It contains an offset to the seek table packet. This packet must be written before the first audio packet. This packet must be present if the “ST” packet is present and is written after the first audio packet.
| Field | Size (bits) | Value | Comment |
|---|---|---|---|
| Offset | n*8; 0 < n < 10 | Offset from this packet to the seek table packet |
Audio Packet¶
[final] This packet key is “AP”. It contains audio frames. The first frame is a key frame.
| Field | Size (bits) | Comment |
|---|---|---|
| Audio Frames | ? | n (or less if last packet) frames of audio as defined in SH packet |
Seek Table Packet¶
[final] This packet key is “ST”.
| Field | Size (bits) | Value | Comment |
|---|---|---|---|
| Seek Count | n*8; 0 < n < 10 | Number of seek elements in this table | |
| Seek Distance | 4 | 0..15 | Distance between referenced blocks = 2^Value |
| Seek Data | ? |
Format of seek data:
Reference offset for seeking is the musepack magic number
First 2 values are stored using the same code as the packet size code.
Next values are coded as:
code = value(n) - 2*value(n-1) + value(n-2) code <<= 1; if (code < 0) code = -code | 1;
code is sent as golomb code with M = 2^12^.
Chapter-Tag Packet¶
[beta] This packet key is “CT”. It contains a chapter position and associated tag. There is 1 packet for each chapter. When used in a file, all CT packets must be consecutive. They must be the next (group of) packet after Seek Table packet if present at the end of the file, or the last (group of) packet before the Stream End packet else. Chapters are presented by the application in the same order as they appear in the file. When used while streaming, this packet can be inserted between AP packets, and the tag data is valid for the next samples, until a new CT packet is sent.
| Field | Size (bits) | Value | Comment |
|---|---|---|---|
| Sample offset | n*8; 0 < n < 10 | Position of the chapter in samples. In a file from the beginning of the file, in a stream from the last sample before this packet | |
| Chapter gain | 16 | The loudness calculated for the chapter, and not the gain that the player must apply | |
| Chapter peak | 16 | ||
| APEv2 tag | n*8 | APEv2 tag without the preamble { ‘A’, ‘P’, ‘E’, ‘T’, ‘A’, ‘G’, ‘E’, ‘X’ } in the header or footer, preferably without footer. This field is optional. |
Security Packet¶
Checksum (MD5, SHA1) or error correcting code (LDPC). To be defined later. May be better to keep security features external only.
Stream End Packet¶
[final] This packet key is “SE”. The packet size must be 3 bytes. This packet is mandatory and must be the last stream packet. Tags, if present, must be written after this packet.
Streaming¶
[alpha] This file format can be used for streaming. The “SH” block is (can be?) used as a synchronization marker. The decoder will scan for a “SH” block and check its CRC. Once the decoder is synchronized, it will start decoding. It’s up to the streaming server to choose when to send “SH” block. To send meta data while streaming, the Chapter-Tag packet can be used.
Tags¶
[final] No packet must be written after the stream end packet, to allow tagging by other applications. Those tags are global to the file, and define the default values for all the tag fields. The Chapter-Tag packet can redefine the field value for each chapter.
Example file¶
Packets keys and magic number are highlighted:
00000000 4D 50 43 4B 53 48 0F 12 A5 AB 62 08 84 FA C1 40 00 1B 1B 52 MPCKSH....b....@...R
00000014 47 0C 01 00 00 00 00 00 00 00 00 45 49 07 A0 01 17 00 53 4F G..........EI.....SO
00000028 08 82 C2 83 31 00 41 50 82 B2 01 BA A7 36 59 FE BC 7B CD 3E ....1.AP.....6Y..{.>
0000003C 10 3B EF 9B 3A 8E DA 22 0B 64 9A 67 AE EC 99 CB 2A 66 4C 79 .;..:..".d.g....*fLy
... Lots of Audio Packets
005081C4 BF FF FF 85 03 81 B7 32 A0 2E 3A E0 D4 FC 20 16 A0 40 80 53 .......2..:... ..@.S
005081D8 54 81 01 47 12 E8 58 21 73 01 83 DC A8 1D 95 4B D9 F5 37 EF T..G..X!s......K..7.
005081EC A9 1E AB 86 CA 3A 1E 12 B7 F4 9A 2A C4 76 84 13 79 95 09 FA .....:.....*.v..y...
00508200 AB D1 86 7C 53 0D BF 84 E9 B3 3F 42 13 EB 02 EE A8 15 CC 79 ...|S.....?B.......y
00508214 20 01 5C 41 0F 21 2A 99 27 78 A6 E8 45 BB 67 A3 10 DE 45 5E .\A.!*.'x..E.g...E^
00508228 8F 38 6B E2 5C 6E 44 09 86 E0 E6 B7 B3 77 67 80 21 04 BF 20 .8k.\nD......wg.!..
0050823C C7 FC BD 9D 77 A5 4D 8C C5 38 38 F6 8D 52 2B FC 56 43 D5 5A ....w.M..88..R+.VC.Z
00508250 AE 2F AD B9 A2 51 D1 D0 53 45 03 ./...Q..SE.