ACCEPTANCE OF AUDIOVISUAL QUALITY IN ERRONEOUS TELEVISION SEQUENCES OVER A DVB-H CHANNEL

In Digital Video Broadcasting for Handhelds (DVB-H), an audiovisual service is transmitted in time-slicing bursts to achieve power savings in receiving terminals. As in many other transmission schemes, the received data streams over DVB-H may contain residual transmission errors. The burst characteristics and appearance frequency of these transmission errors have an effect on the perceived audiovisual quality. The perceived quality is a significant factor in the end-user’s acceptance and the adoption of new mass products such as mobile television. This study investigated effects of four residual time-slice error rates on the overall subjective audiovisual quality. The quality was measured with subjective retrospective ratings of acceptance. Error rates of 1.7%, 6.9%, 13.8% and 20.7% were tested with clips from four popular television programs and evaluated by 30 participants. The results show that among the error rates simulated, error rates of 1.7% and 6.9% were experienced as acceptable, while error rates of 13.8% and 20.7% were perceived to be unacceptable. 1. ACCEPTANCE AND DVB-H Mobile television is expected to become a popular ubiquitous media service. Market research [14], performed to survey the viability of mobile television, showed encouraging trends and a possibility of high acceptance and usage of this service. However, due to the error-prone transmission environment, mobile television also presents substantial challenges in the context of user acceptance to erroneous multimedia streams. In error-prone environments, burst errors and their frequency in the transmission channel have an effect on subjectively perceived audiovisual quality. This perceived quality plays a significant role in user’s acceptance of the new media services. In the larger perspective, it also has an effect on the adoption of new mass products capable of mobile television. Therefore, it is important for service and content providers to understand the relation between subjective acceptance of quality and the objective thresholds of transmission errors. Acceptance The acceptance threshold is the minimum quality level that fills the requirements of subjectively perceived quality. Here, quality refers to subjective quality, based on perceptual evaluation and processes. These processes combine low-level sensation and high-level cognitive processes [1]. In subjective evaluation, acceptance threshold is essential to study for finding the lowest beneficial level of perceived quality. The published studies (e.g.[15][16]) and the current research methodologies recommendations [9][10] focus on finding the quality preferences instead of the measurements of acceptance. DVB-H Digital Video Broadcasting for Handhelds (DVB-H) [4] is the latest initiative by the Digital Video Broadcasting (DVB) organization to enable point-tomultipoint type broadcast/multicast transmission to lowpowered hand-held devices. In order to achieve power savings in receiving terminals, DVB-H uses a timesliced data transmission scheme, in which the data packets of an audiovisual service are transmitted in bursts that occur every few seconds. This allows receivers to switch off the radio reception while incoming data bursts are not expected. Like any typical wireless channel, the DVB-H channel is also prone to transmission errors. Hence, to enable error resilient data transmission, DVB-H uses a Reed-Solomon forwarded error correcting (FEC) code, computed over a timeslicing burst. DVB has finalized the selection of the audio and video codecs to be used in the IP data casting service over DVB-H [5]. The Advanced Video Codec (H.264/AVC) [7] and the High-Efficiency Advanced Audio Codec version 2 (HE AAC v2) [8] are the recommended codecs for the service. The specification also defines three Internet Protocol Integrated ReceiverDecoder (IP-IRD) classes for DVB-H, each one specifying a certain decoder operation point in terms of picture size and required decoding data rate. The relation between transmission errors with different error characteristics and subjective quality is a largely unexplored domain, especially, in the context of DVB-H. The subjective experiments have focused on testing compression errors (low bitrates, framerates and screen sizes) relevant to mobile video presentation (e.g. [15][16]). Related to experience of transmission errors, Pastrana-Vidal et al. [12][13] have published about detection thresholds in sporadic signal loss. Their studies concentrated separately on audio and video signal loss with short clips (10 sec.). Dependently on content, its activity and the duration of the signal loss the auditory detection threshold varies from 1ms to 6ms. The visual detection threshold is 80ms. However, the length of discontinuity duration of 30ms is audible in all contents and in video the unequivocal detection rate is 200ms. It is also summarized that in the visual sporadic frame dropping is more annoying in the case of several short discontinuities than a long lasting single burst. As illustrated, there is a need of studying the effect of realistic transmission errors on perceived quality; the effect of error rate in DVB-H on subjective quality in terms of acceptance is presented in this paper. 2. RESEARCH METHOD AND TEST SET-UP This study examined the effect of error rate on subjective acceptance of audiovisual quality presented with a mobile device. The acceptance was measured with a retrospective rating method, modified from ITU’s recommendations [9][10]. Testing took place during autumn 2005. Participants: Altogether 30 evaluators (6 professionals; 24 non-professionals), aged 18-45 representing equal number of both genders participated to the test. The number of innovators and early adopters was restricted to 20% and technophobes and laggards were screened out. All participants had normal or corrected-to-normal visual acuity and normal hearing acuity. Test procedure: Each experimental session started with vision and hearing tests with demographic data collection. Thereafter, the combined training and anchoring took place. Participants were shown the extremes of the sample qualities as an example of the quality scale and to familiarize them to the test and used contents. In the test, the sample material was shown using the single stimulus method where clips are viewed one by one and rated independently. During each clip presentation (duration 60 sec), unacceptable quality was indicated by pressing a button of a game controller. After each clip presentation, participants had five seconds to mark the quality score of a clip on an answer sheet by using a discrete, unlabelled scale from 0 to 10 and the acceptance of the clip (yes/no choice). All clips were played three times and the positions of transmission errors varied in each repetition. The posttest session focused on gathering qualitative data of experiences on erroneous streams. One testing session lasted for about 1.5 hours. Test material selection: The clips used in the experiment were selected according to their popularity, potentiality for the mobile TV content, variations in spatial details and temporal motion and audiovisual characteristics [15]. Animation, music video, news, and sport sequences were used in the clips that contained a meaningful segment of a TV program without annoying cuts off at the beginning or end of the clip. All clips also had at least moderately high level of details and temporal motion as well as textual information. Animation, news, and sport sequences contained only speech, whereas the audio track of the music video clip included both musical instruments and vocals. The clip duration of 60 sec guaranteed that at least one error appeared in one of the repetitions with lowest error rate. Test material production process – Simulation: Audio and video streams according to the lowest specified IP-IRD were encoded. QCIF videos were coded with an H.264/AVC encoder at a bit rate of 128 kbps and at a frame rate of 12.5 Hz. At least one instantaneous decoding refresh (IDR) picture was encoded per each time-slicing burst to enable new recipients tuning in. Monaural audio at 32 kbps and 16 kHz sampling frequency was coded with an AAC encoder (providing a subset of the features of the recommended HE-AAC v2 audio encoding for the IP datacasting service). For simulating a DVB-H channel transmission, each encoded audio-visual sequence was encapsulated into Internet Protocol (IP) packets by adding the IP, User Datagram Protocol (UDP) and Real-Time Transport Protocol (RTP) headers. Each video IP packet contained one Network Abstraction Layer (NAL) unit of H.264/AVC video, which typically corresponds to one picture, and each audio packet contained one frame of encoded audio. The audio and video streams of an audiovisual service were multiplexed and encapsulated into the same time-slicing bursts and several audiovisual services were further encapsulated into the same timeslicing bursts such that the burst interval was approximately 1.5 seconds. Channel error patterns for the DVB-H channel were created based on field tests, which were carried out in an urban area with an operable DVB-H service system. The receiving device was located in a moving car. The Multi-Protocol Encapsulation Forward Error Correction (MPE-FEC) method, specified in [3], was used as an additional error protection mechanism. Accordingly, the IP packets of a time-slice were collected column-wise into the application data table (ADT) of an MPE-FEC frame, and Reed-Solomon FEC codewords were calculated row-wise into the ReedSolomon data table (RSDT). The numbers of columns were 191 and 64 in ADT and RSDT, respectively, and the number of rows in the MPE-FEC frame was 512. An analytical approximation of the field-tested DVB-H system was then generated using the popular GilbertElliot (GE) channel error model [2]. The field tests resulted into an MPE-FEC error pattern, which was used to train the model and estimate the parame