Error concealment in digital audio receiver

#The invention relates in general to detection and concealment of errors in a signal transmitted in digital form from a transmitter to a receiver. In particular the invention relates to detection and concealment of transmission errors in an audio signal processed in the form of frames by a digital audio receiver.

#Transmission of an audio signal in digital form from a transmitter to a receiver is known as such and it is going be become more common as digital television and broadcasting systems replace older systems based on analog frequency modulation. Known telecommunications standards dealing with the transmission of digital audio signals include the ETS 300 401 standard by the European Broadcasting Union (EBU) and European Telecommunications Standards Institute (ETSI) and the ISO/IEC 11172-3 and ISO/IEC 13818-3 standards by the International Standard Organization (ISO) and International Electrotechnical Commission (IEC). These standards specify a certain frame structure for the transmission of a digital audio signal. The ETS 300 401 standard, which is also called the DAB (Digital Audio Broadcasting) standard, specifies a frame structure which in a way is a special case of the frame structure specified in the ISO/IEC 11172-3 and ISO/IEC 13818-3 standards as it contains additional specifications concerning frame structure particulars left open in the earlier standards. With an audio signal sampling frequency of 48 kHz the DAB standard is based on the ISO/IEC 11172-3 standard and with a sampling frequency of 24 kHz on the ISO/IEC 13818-3 standard. To illustrate the background of the invention, the structure of the audio frame according to the aforementioned standards and its processing in transmitter and receiver apparatuses is described in brief below.

#FIG. 1 is a simplified block diagram of an apparatus 1 according to the ISO/IEC 11172-3 and 13818-3 Layer II standards generating DAB frames from a pulse-code-modulated (PCM) audio signal. The apparatus comprises an input port 2, output port 3, and between them, a filter bank 4, quantising and coding block 5, and a frame generating block 6, connected in series. In parallel with the filter bank 4, there is a psychoacoustic model block 7 the input signal of which is the same as the filter bank input signal. The outputs of blocks 4 and 7 are taken to a bit allocation block 8 the output of which controls quantising and coding in block 5. The apparatus also comprises a data port 9 such that digital program associated data brought thereto is taken to the frame generating block 6 which incorporates the program associated data in the frame structure.

#The invention is described in more detail with reference to the preferred embodiments presented by way of example and to the accompanying drawing wherein

#FIG. 1 shows a known encoder,

#FIG. 2 shows a known decoder,

#Above in conjunction with the description of the prior art reference was made to FIGS. 1 to 4, so below in the description of the invention and its preferred embodiments reference will be made mainly to FIGS. 5 through 8. Like elements in the Figures are denoted by like reference designators

#In the method according to the invention, the receiver uses the data contents of several successive frames to decode the frame being processed at a given time and to conceal the errors possibly detected in it. FIG. 5 shows a ring-like frame table 42. The shape of the table as such is of no concrete significance because in a preferred embodiment of the invention it only exists as a certain number of computer memory locations, but since so-called cyclic pointing is advantageously used for pointing to the one-frame blocks 42a in the table, it is illustrative to present the table in a ring-like form. Cyclic pointing means that a given block, having the address [k], is followed in the table by a block the address of which is [(k+1) mod NFRMS], where NFRMS is the number of blocks in the table. The receiver according to the invention initially stores each received frame in the table 42 according to FIG. 5 in the form which the frame has as it arrives at the input port of the decoder.

#FIG. 5 also shows a window 43 used by the decoder of the receiver to decode the frames and to detect and conceal the transmission errors possibly occurring in them. Size of the window is an integer number of frames greater than zero. The index of the frame in the middle of the window, which identifies the frame within the window, is 0, and the frame is called the current frame. Those frames in the window that have been received and stored in the table 42 after the current frame are successor frames and the one farthest away from the current frame is the front frame. Those frames in the window that have been received and stored in the table 42 before the current frame are predecessor frames and the one farthest away from the current frame is the rear frame. The number of successor frames is marked cnnxt (from “current number of next frames”) and the number of predecessor frames is marked cnpre (from “current number of previous frames”). The values of cnnxt and cnpre can change dynamically in a manner which will be described in more detail later on, but they must satisfy the double inequality 0≦(cnpre+cnnxt)<NFRMS, for the size of the window 43 in frames (=cnpre+cnnxt+1) to be always at least 1 and not more than NFRMS. If the size of the window 43 is one frame, the names front frame, rear frame, and current frame all mean one and the same frame.