/// <summary>
/// Decodes from a header packet if the service is being transmitted serially, and signals a header to the magazines if it is.
/// </summary>
/// <param name="packet">The teletext packet to be decoded.</param>
private void DecodeHeader(Packet packet)
{
// Get the serial flag
bool magazineSerial = false;
byte controlByte = Decode.Hamming84(packet.Data[7]);
if (controlByte != 0xff)
{
magazineSerial = Convert.ToBoolean((byte)(controlByte & 0x01));
}
// If the flag is set, loop through each magazine and signal to them that a header has been received
if (magazineSerial)
{
for (int i = 0; i < 8; i++)
{
// Only signal a header has been received if the magazine is not the one for this header
if (i + 1 != packet.Magazine)
{
Magazine[i].SerialHeaderReceived();
}
}
}
}
/// <summary>
/// Initialises a new instance of the <see cref="T:TtxFromTS.Teletext.Packet"/> class.
/// </summary>
/// <param name="packetData">The teletext packet data to be decoded.</param>
public Packet(byte[] packetData)
{
// Store the original full packet data
FullPacketData = packetData;
// Retrieve the field
Field = !Convert.ToBoolean((packetData[0] >> 2) & 0x1);
// Retrieve the line number
LineNumber = (byte)(Decode.Reverse(packetData[0]) & 0xF);
// Retrieve the framing code
FramingCode = packetData[1];
// Check the framing code is valid, otherwise mark the packet as containing errors
DecodingError = FramingCode != 0x27;
// Retrieve and decode the magazine number
byte address1 = Decode.Hamming84(packetData[2]);
Magazine = address1 & 0x07;
// Check the magazine number is valid, otherwise mark the packet as containing errors, and change 0 to 8
if (Magazine > 7)
{
Magazine = null;
DecodingError = true;
}
else if (Magazine == 0)
{
Magazine = 8;
}
// Retrieve and decode the packet number
byte address2 = Decode.Hamming84(packetData[3]);
Number = (address1 >> 3) | (address2 << 1);
// Set the packet type from the packet number, or if it is not a valid number mark the packet as containing errors
switch (Number)
{
case 0:
Type = PacketType.Header;
break;
case int packetNumber when packetNumber <= 23:
Type = PacketType.PageBody;
break;
case 24:
Type = PacketType.Fastext;
break;
case 25:
Type = PacketType.TOPCommentary;
break;
case 26:
Type = PacketType.PageReplacements;
break;
case 27:
Type = PacketType.LinkedPages;
break;
case 28:
Type = PacketType.PageEnhancements;
break;
case 29:
Type = PacketType.MagazineEnhancements;
break;
case int packetNumber when packetNumber == 30 && Magazine == 8:
Type = PacketType.BroadcastServiceData;
break;
default:
Number = null;
DecodingError = true;
break;
}
// Retrieve packet data
Data = new byte[packetData.Length - 4];
Buffer.BlockCopy(packetData, 4, Data, 0, packetData.Length - 4);
}
/// <summary>
/// Decodes information from a broadcast services data packet.
/// </summary>
/// <param name="packet">The teletext packet to be decoded.</param>
private void DecodeBroadcastServiceData(Packet packet)
{
// Get the designation byte
byte designationByte = Decode.Hamming84(packet.Data[0]);
// Check the designation byte does not have unrecoverable errors, and if it does ignore the packet
if (designationByte == 0xff)
{
return;
}
// Decode the designation code and multiplex status
bool multiplexed = Convert.ToBoolean(designationByte & 0x01);
int designation = designationByte >> 1;
// Check the designation code is 0 (format 1) or 1 (format 2), otherwise ignore the packet
if (designation > 1)
{
return;
}
// Set the multiplexed status
Multiplexed = !multiplexed;
// If the packet is format 1, decode the network identification code
if (designation == 0)
{
byte[] networkID = { packet.Data[7], packet.Data[8] };
NetworkID = BitConverter.ToString(networkID).Replace("-", "");
}
// Get the status display
byte[] statusCharacters = new byte[packet.Data.Length - 20];
for (int i = 20; i < packet.Data.Length; i++)
{
statusCharacters[i - 20] = Decode.OddParity(packet.Data[i]);
}
StatusDisplay = Encoding.ASCII.GetString(statusCharacters);
// Decode the digits for the inital page number
byte pageUnits = Decode.Hamming84(packet.Data[1]);
byte pageTens = Decode.Hamming84(packet.Data[2]);
// Decode the bytes for the initial subcode
byte subcode1 = Decode.Hamming84(packet.Data[3]);
byte subcode2 = Decode.Hamming84(packet.Data[4]);
byte subcode3 = Decode.Hamming84(packet.Data[5]);
byte subcode4 = Decode.Hamming84(packet.Data[6]);
// If the subcode bytes containing the magazine number contains errors, stop processing
if (subcode2 == 0xff || subcode4 == 0xff)
{
return;
}
// If the page number digits don't contain errors, set the initial page number
if (pageUnits != 0xff && pageTens != 0xff)
{
// Get the magazine number, changing 0 to 8
byte magazineNumber = (byte)((subcode2 >> 3) | ((subcode4 & 0x0C) >> 1));
if (magazineNumber == 0)
{
magazineNumber = 8;
}
// Set the initial page number
InitialPage = magazineNumber.ToString("X1") + ((pageTens << 4) | pageUnits).ToString("X2");
}
// If the remaining subcode bytes don't contain any errors, set the subcode
if (subcode1 != 0xff && subcode3 != 0xff)
{
byte[] fullSubcode = { (byte)(((subcode4 & 0x03) << 4) | subcode3), (byte)(((subcode2 & 0x07) << 4) | subcode1) };
InitialSubcode = BitConverter.ToString(fullSubcode).Replace("-", "");
}
}
