Multichannel audio on television, under debate

The generalization of multichannel audio on television is posing new challenges from a technological and creative point of view that numerous experts have analyzed in Madrid in a seminar organized by the RTVE Institute in which numerous experiences in immersive audio have been put on the table.

The generalization of multichannel audio on television is posing new challenges from a technological and creative point of view. To analyze the implementation, production and transmission of multichannel audio in broadcast environments, the RTVE Institute has organized a seminar this April 26 in Madrid in which prominent professionals have participated who have provided an overview of the situation by putting numerous implementation examples on the table.

Ricardo Alvariño, Deputy Director General of Infrastructure and Technical Regulations of the Ministry of Industry, opened the floor by inaugurating this seminar, stating that "audio is sometimes, unfortunately, in second place, after video, forgetting its importance. Now, with digital television we have new possibilities for distributing 5.1 audio with notable advantages." Alvariño has emphasized the importance of coordination between program and signal producers and receiver manufacturers for correct reception of multichannel audio.

For his part, Suso Ramallo, president of AES Spain, has highlighted the importance of multichannel. "We hope that, after the emergence of high definition, HD will also reach audio and, without a doubt, in the near future we could talk about spatial coding by objects, opening up new ones, until now unknown, to creativity."

Pere Vila, director of Planning and Technological Innovation at RTVE, has made a call to take advantage of this day to debate and share experiences in engineering, network, production... both by producers, operators and broadcasters.

The first presentation was given by Juan Carlos Díez, head of the El Corte Inglés recording studios, who focused his intervention on the analysis of the work and recommendations of the EBU and other relevant entities on multichannel audio and its application in the field of television and radio production and broadcasting.

Díez has made a curious parallel between Darwin's theory of evolution and the evolution of audio from the monkey (monophonic audio) to a robust species with spatial development, the Neanderthal (the stereophonic one) and now, Homo Sapiens, characterized by its skills, sociability and curiosity. The multichannel stereophonic sound system, based on ITU-R BS.775 (last revised in 2006), would represent this last evolutionary stage marked by sociability and respect for the norm.

What does BS.775 recommend? Adopt the channel hierarchy, establish a reference speaker arrangement and use five signals (left, right, center, left and right surround, and a sixth signal that would be the LFE-Low Frequency Extension), preserving backward compatibility with the audio systems that have existed until now. This entire framework, according to Juan Carlos Díez, should be a universal and recommended reference.

Díez has reviewed the importance of calibrating the rooms to have a reference listening place and the correct assignment of recording channels following the UER R.91 recommendation of 1999, thus facilitating the exchange between producers and broadcasters.

With respect to the transmission of what was recorded, the workhorse of transmission is the limited bandwidth available. "Faced with matrix coding, with the impossibility of broadcasting in simulcast, current perceptual coding systems now manage to compress information 12 to 14 times, although losing information in the process, but without the listener being able to perceive it," said Díez, also reviewing the different perceptual coding formats, both open codecs (MPEG-1 Layer II, MPEG-2 BC Layer II, HE-AAV, HE-AAC v2…) as proprietary such as Dolby or DTS.

However, it has ensured that Dolby E is the only one capable of meeting the user requirements for audio coding systems in broadcasting (ITU-R BS1548-2).

How many DTT stations are capable of broadcasting multichannel? Of the 31 channels with national coverage in Spain, only 8 use MPEG-2 prepared to broadcast multichannel. In radio, DAB would not allow multichannel, unless DAB+ (HE-AACv2) was used.

In Europe, since 2005, multichannel programs are frequently broadcast with pioneer stations such as ORF, BBC, ZDF or SVT. Both movies, series and sporting events are usually the most widespread broadcasts in multichannel audio.

However, according to Díez, in Spain in this aspect we are behind Europe. TV3 was a pioneer by broadcasting a movie in 5.1 on Fridays, as well as the Champions League and Formula 1. TVE recently began some experience in Dolby Digital Plus on its high definition channel, while Radio Nacional broadcasts a concert in this format weekly. On payment platforms, multichannel audio broadcasting is widespread.

Effects on engineering

Luis Santillana, head of audio frequency at RTVE, has focused his intervention on the effects on the overall architecture of the facilities and those aspects to take into account in production centers, controls and continuities.

Santillana has highlighted a series of recommendations to take into account in the conditions of the room where the mixing is going to be carried out despite the fact that sometimes, such as in mobile units with restricted space, "we find rooms that separate us from ideal listening conditions."

In addition, there are other necessary elements, he assured, "for the sound system and control of a television program, which make difficult the positioning of the speakers conditioned by the control fish tanks or booths, pre-program monitors or auxiliary cameras, computer equipment, technical racks... thus interfering with the perfect arrangement for listening. The solution: the best compromise between space, technical needs and recommendations, with the greatest possible adjustment between engineering and the final operation."
At TVE they use discrete microphones for multichannel pickup for sound pickup and surround sound pickup systems with 5.0 cannons, Zepelin 7.1 type microphones (configured in 5.1), three-dimensional systems for audio post-processing (B-Format), portable multi-channel audio recorders-mixers on hard disk...

Regarding the studio and mobile unit, Santillana has highlighted the importance of having PPM, VU and loudness measurement equipment integrated into the sound desk, as well as surround coherence analyzers that allow for a more in-depth analysis of the frames.

In digital tables, the current tables present a network topology with vertical control of the 5.1 signal, also allowing the management of multiple multi-channel monitoring.

In playback, TVE works with speakers with DSP processing, discrete systems for quality control and mini-monitors for visualization and control of multichannel audio, as well as headphones with binaural processing for 5.1 listening.

Finally, Santillana has addressed the voice-over experience with the help of an automatic mixing system for multichannel audio, with center channel compensation between the 5.1 version and stereo, use of Lo/Ro for sports and meta configuration (to help correct the decompensation of 5.1 with respect to stereo when reducing channels).

TVE has a specific mobile unit for the capture and post-production of programs with 5.1 surround sound. The HD mobile units are also prepared for 5.1, as well as Studios 10 and 11 in Prado del Rey (Madrid), the Teledeporte studios and 5 and 6 in Sant Cugat (Barcelona) and A4 in Torrespaña... the latter have been a recent action ahead of the Olympic Games. Post-production and dubbing studios and editing rooms are also fully prepared to work in a multichannel environment.

On the other hand, David Vivas, technical director of Unitecnic, has explained the effects on systems engineering in the implementation of workflows with multichannel audio for a better experience for the viewer. Vivas has emphasized the working modes, types of signals and files that multichannel implies.

"The production, post-production, archive and broadcast environments must be prepared and equipped to produce, process, archive and transmit multichannel signals. We will need to be able to have a workflow that allows operating with multichannel signals, being compatible with the reception of this signal in the homes of the final consumer. It is important to ensure the compatibility of multichannel broadcasts in reception, controlling how the transition from six reception channels to a reproduction in two or one channel is going to be carried out with the help of metadata," he said. insured.

"Ideally we should move in the area of ​​signals in which we usually operate. The use of SDI with embedded audio and AES for audio signals guarantees a familiar environment. The files must support the use of multichannel audio content and ideally, as a widely implemented format, but without ruling out others. Thus, we should be able to use MXF," says Vivas.

According to Vivas, it is not strictly necessary to use multichannel microphones, while in mixing, in addition to the console (which can even be a classic stereo one), he has highlighted the need to have a monitoring system that allows us to listen to our program as it is being produced as well as adjust the metadata within the Dolby E encoding or in SDI as auxiliary data if we have embedders that support this feature. As far as meters are concerned, it is important that they comply with R128 since it will be necessary to measure loudness not only electrical level.

In post-production environments, Vivas has highlighted that “it is important that post-production systems can work in shared storage environments to optimize workflows to access materials from different points.”

In central control and continuity, "in these rooms, due to their size and shape, as well as the number of signals they control, it is difficult to optimally adapt them to monitor 5.1. Therefore, it is advisable to have a little courage within the same control to install a 5.1 listening set to use it sporadically," stated Vivas.

Using Dolby E as a contribution and distribution format, switching systems, servers, and modular equipment must be compatible with this type of signal.

David Vivas has outlined that Dolby Digital Plus is an audio coding format for broadcast transmission, not just multichannel, supporting from mono to 7.1 with a very high coding bit rate optimization.

Adjusting the bandwidth

Miguel Ángel Bona, director of radio communications planning at RTVE, has reviewed some general considerations and parameters and has shared RTVE's specific experience in this matter.
Bona has highlighted the pioneering work being carried out by Radio Clásica HQ when in December 2011 it launched the first multi-channel audio broadcast on radio in Spain. Since then, every Friday it broadcasts a concert in 5.1 (within RTVE's second DTT multiplex, with Dolby Digital AC3+ encoding).

HQ Classic Radio, to ensure compatibility with all receivers, emits two audio streams simultaneously: a stereo stream encoded in MPEG1-Layer II and a second encoded with AC3+ with a stereo or 5.1 option. In both cases, at 320 Kb/s, 16 bits, 48 ​​KHz.

“AC3+ allows us to include a variable number of audio channels, with a considerable reduction in data speed compared to AC3 with a speed between 32 Kb/s to 6 Mb/s, with metadata support,” stated Bona.

Another experience worth highlighting in multichannel by the Spanish public broadcaster is TVE HD, conditioned in this case to the fact that the bit rate of the Spanish multiplexes is only 19.9 Mb/s. TVE HD broadcast for the first time with surround sound on January 25 on the occasion of a match between Spain and Slovenia in the European handball tournament. In this case the same system is used as in Radio Clásica HQ.

The new DTT headend encoders in Torrespaña allow input in AES/EBU (PCM) or Dolby E format (up to 8 audio tracks per AES/EBU pair). The encoder allows transparent switching from one format to another, even between tracks, in the absence of Dolby E.

The HQ Classical Radio experience

Miguel Ángel Coleto, director of RNE broadcasts and media, has reviewed Radio Clásica's experience in surround sound broadcasting.

Coleto has reported that in a previous phase a series of tests were carried out at concerts and an intense exchange of opinions with engineers from ORF, NDR or RTBF..., before the first official broadcast that took place on December 2, 2011. In this experience, Radio Nacional de España has chosen to be faithful to the main pattern of the stereo mix with expansion to 5.1 through a surround panning of signals, compensating for the downmix with attenuations of the center and left channels and right rear with -3 dB and -6 dB, respectively.

In pickup, RNE relies on a multi-microphone array combined with an adapted Decca surround tree in its 5.1 concerts.

Coleto has also highlighted the need to transport, process and encode these signals through the RTVE contribution network, counting on new multichannel coding equipment at the Torrespaña headwaters. As it does not have a traditional radio system capable of supporting these broadcasts, Radio Clásica broadcasts its programming through DTT, ensuring compatibility in the broadcast of Radio Clásica HQ between the MPEG1 Layer III signal and Dolby Digital Plus with a continuous transmission of the two encodings.

To carry out this broadcast, RTVE has seen the need to adapt the headend of the second multiplex, as well as the transport chain, central control and continuity of Radio Clásica. Coleto has emphasized the simulation of emission and reception of both services (stereo and Dolby E) on different types of DTT receivers and televisions to guarantee reception compatibility in all possible scenarios.

From now on, RNE faces new challenges such as multichannel live broadcasting, expanding the production of events with surround sound to other stage spaces and festivals. In addition, the adaptation of studios and production and post-production means will be expanded.

Coleto has assured that the challenge now lies in researching and promoting, through the development of digital radio, a means of broadcasting with multichannel audio, as well as provoking in listeners a perceptual stimulus capable of recognizing the progress that surround sound means with higher quality products.

Metadata

Gabriel Solsona, head of TVE's Sound Unit, has analyzed the pros and cons of .0 and .1 production. The 5.1 provides greater creativity, ease of transcription and difficulty of managing 6 channels. For its part, the .0 offers a more basic deployment, without any compromise in broadcast, as well as greater ease of control of 5 homogeneous channels.

Solsona has highlighted the importance of metadata in a multichannel environment when it comes to recognizing material from archives or facilitating workflow in production and broadcast. The metadata gives us the level of loudness normalization that a signal carries, what multichannel format it has, where that signal comes from...

Dolby E has solved a large part of the management of this metadata in accordance with the SMPTE 2020 standard, Solsona highlighted.

To handle simultaneous mixes (stereo and surround), it is possible to do so in an attended, semi-attended and unattended manner. Both mixes can be carried out in parallel or through a downmix from 5.1 to stereo, or, finally, generate a single mix that, thanks to the metadata, allows when a consumer can only access stereo, the downmix is ​​generated at a well-served destination thanks to the metadata. This would be the trend in the future, according to Solsona.

Canal + HD

Juan Piñar, ingeniero de Proyectos de Prisa TV, ha expuesto en esta jornada la experiencia de un operador privado como Canal+ que emplea en la emisión de su canal Canal+ HD audio multicanal en Dolby Digital con dos programas de audio en multicanal (versión española y original).

Cabe destacar que PRISA Tv ha apostado por un total respeto a los contenidos originales sin llevar a cabo un upmixing así como por la compatibilidad plena de contenidos en alta definición en la emisión en la versión en definición estándar (en estéreo) sin generar un downmix. Todo ello conlleva trabajar con un total de 16 canales de audio gracias a un empaquetado con Dolby transportándolos embebidos en el stream de vídeo SD.

PRISA Tv trabaja con equipamiento de decodificación y codificación, con equipo modular, audio embebido (I/O solamente de vídeo) y compensación de retardos internamente.

El trabajo con Dolby E ha significado, por otro lado, ampliar los controles de calidad para vigilar posibles errores CRC, la alineación de la trama de audio con el intervalo vertical de vídeo o el uso de los metadatos con parámetros de codificación correctos. PRISA ha optado por transportar los metadatos del contenido original al receptor del abonado con paquetes de datos diferentes para la versión española y la original.

En cuanto a archivo, Piñar ha hecho hincapié en la necesidad de decodificar las dos tramas Dolby E archivando cuatro pares de audio (V.E. Dolby E 5.1+2, V.O. Dolby E 5.1+2, V.E ST y V.O. ST).

En postproducción, PRISA Tv decodifica la trama Dolby E en la propia sala de vídeo mientras que en la sala de audio se trabaja con los audios del proyectos de vídeo ya decodificados.

Esta jornada sobre audio multicanal la ha cerrado Fernando Martín de Pablos, director y cofundador de EDSOL Records, quien ha hecho un repaso sobre el sonido envolvente en el ámbito de la postproducción podiendo numerosos ejemplos de las diferentes técnicas de Upmix y Downmix que utilizan en sus estudios para pasar de estéreo a multicanal y viceversa. Martín de Pablos ha compartido también con la sala otros ejemplos de sonoridad multicanal y estéreo.

Segunda Jornada sobre Audio Multicanal

Este seminario se complementará con una segunda jornada convocada por la Universidad Rey Juan Carlos I que tendrá lugar el miércoles 9 de mayo en el Campus de Fuenlabrada (Madrid) dirigida por Manuel Sánchez Gil.

En las ponencias de la mañana, se tratarán temas como la implementación tecnológica básica para la producción con 5.1 y la innovación conceptual en este sistema de sonido, entre otros. Por la tarde, el programa estará formado por doce talleres que abarcarán desde los costes de implementación para la producción de audio multicanal hasta el diseño de sonido 5.1 en cine, pasando por la producción musical en multicanal o la creatividad y producción de sonido en surround.