Soundrangers creates all original high quality sound effects and royalty free production music for movies and film and deliveries these sound effects and music from this website. This is a little how-to from our years of experience working on custom sound design for movies and film. This guide is meant to give both independent film makers and audio engineers a quick overview of providing sound for film using digital audio and today's post production techniques.

A Brief History of Time Code


Before talking about the actual steps involved in post production sound for film in the world of digital audio, it will help to have a good understanding of time code and how it relates to sound for film. The terms and concepts explained in this chapter will be referred to throughout the rest of this booklet. Time code's full name is SMPTE time code, named after the Society of Motion Picture and Television Engineers. This is the current industry standard method of synchronizing audio, video, and film. Basically time code is just a sophisticated tape counter referencing time. It's a series of numbers recorded in conjunction with audio, video, and film. Time code is usually recorded onto the tape or film itself, or in the case of computer files it is encoded into the sound file.

These numbers act as both a running speed reference and a position reference that regulate the playback of your material. Time code ensures that your material is running at the correct speed while helping you to know exactly where you are within your material. Simply put, how fast the numbers are going by determines running speed, and where you are within those numbers determines position. This time code information is used to regulate tape machines, film, computers, and just about any other media playback device capable of reading it.

Time code gets its origins from the pilot tone, an older method of synchronization which is still in use today on some older non-time code friendly machines. This method consists of a 60Hz frequency or pilot tone that is generated from common AC wall current. This pilot tone is then recorded onto tape in conjunction with the program material. During playback the 60Hz pilot tone on the tape is compared to another 60Hz signal to ensure that the tape is running at the correct speed. While this method can regulate the running speed, it cannot provide any position reference. That's why there is a count down with a sync sound on the number 2 [2 pop] at the beginning of films. The 2 pop acts as a position reference. If the audio tape machine and the picture tape machine are both running the same speed based on a pilot tone and the 2 pops on both machines are lined up, then theoretically the two machines are in sync and will stay in sync. Time code however, because of the running numbers, can continually check speed and position throughout the entire length of a program.

The original time code standard was 30 frames per second [fps], (the 30 frames came from dividing the 60Hz frequency in half). Machines referencing 30fps time code were designed to run at the same speed as those referencing a 60Hz pilot tone. 30fps time code also counts the same as real time or matches the clock on the wall (except for the actual frame numbers). Meaning time code hours, minutes and seconds are all the same and match actual time.

With both time code and pilot tone referenced machines running at the same speed, everybody was happy. The problem is that 30fps became the standard for black and white TV--then color TV came along and changed everything. To accommodate the additional information needed to produce color the picture frame size was increased. Consequently, only 29.97 of the 30 picture frames could be played in one second. This meant the running speed of color video had to be slowed down .1% to allow all 30 frames of color video to be played with the standard television scan rates. This .1% slow down means that it runs roughly 2 frames per second slower. So instead of 30fps you only get 29.97fps and the 60Hz reference frequency becomes 59.94Hz. Of course color TV become the standard, so consequently the running speed of color video also became the standard. Older formats such as film had to conform to this new slower speed in order to be compatible.

Over the years other variations in time code types were developed to accommodate different formats [see the Time Code Chart].

The term 'frame rate' is used to describe the different types of time code. Frame rate includes two things: how many frames per second there are, and whether or not the numbers count consecutively. There two main types of time code, drop frame [df] or non-drop frame [ndf]. Since 29.97fps runs slightly slower than real time, drop frame was developed. Drop frame actually runs at 29.97fps it just appears as through it runs at 30fps. Drop frame strategically drops or leaves out frame numbers (108 frame numbers every hour) so that the time code hours, minutes and seconds match real time. It's really just a re-numbering of 29.97ndf. Network television likes this frame rate because the number read-out does match real time, so the exact program length can easily be determined. With Non-drop frame, the frames are numbered consecutively, meaning it does not drop any frames. This is a little easier to work with than Drop frame because you don't have to think about the missing frame numbers. Non-drop frame is the standard for everything other than broadcast programming.

Film is unique in that it predates time code so the pilot tone and 2 pop still apply, but it also has the capability to interact with time code so that applies as well. Film is a unique format in that understanding old and new synchronizing techniques and how they interact can be very helpful. Working with time code can be a nightmare or a beautiful experience depending on how well you understand it. Either way time code can't be avoided; it is an integral part of all phases of production.

7 Steps to Sound Success


Step 1:
Transfer Film to Video

There are two main issues that need to be addressed when transferring film to video. The first issue is the fact that film runs at the old standard speed which is .1% faster than NTSC color video-the current industry standard. Since video's running speed is the industry standard, all the synchronization gear in recording studios reference this, i.e., run at this speed. This means film usually has to conform to this speed in order to work with sound and video in most post production studios. The second issue is: film has 24 picture frames per second as compared to color video's 29.97 picture frames per second. To help deal with these two differences, a transfer process known as a 3:2 pull down was developed. During a 3:2 pull down, film is slowed down .1% to video speed and the 24 picture frames per second of film are converted to video's 29.97 picture frames per second. Whenever film is transferred to video, it must go through a 3:2 pull down.

In order to work on post production sound for film with a Digital Audio Workstation [DAW] the film must be transferred to video. This transfer can come either before or after the film is edited. If the film is going to be edited on a flatbed, then a copy of the edit is transferred to video. If the film is going to be edited with a digital video editor, then the film will be transferred to video before it's edited. Whatever picture editing method you use, afterward you will need a copy of the edited film transfered to video inorder to be able to work with the sound using a DAW.

It's fine to start the process of adding sound effects and production music with the output of an off-line digital editor, but don't use this copy of the edit as the final sync reference. When a negative is cut using an Edit Decision List [EDL] from a video editor, it's not unheard of to be off an occasional frame or two on a few edits, which can compound and by the end of a film the sync may be off a bit. This can create a noticeable difference between the output of the digital editor and the actual cut film, especially if a video editor not designed to work with film was used. Final audio sync should always be checked against a video made of the copy of the actual cut negative (called an answer print or timing print).

Video prepared adding sound effects and music should contain:
1. A copy of the edited film.
2. Production audio track for reference.
3. Leader with a visual and audible 2 pop.
4. SMPTE Time Code.

The accuracy of the 2 pop and time code at this stage is crucial because these will be the sync references throughout the process of adding sound effects and production music and to transfer the sound back to film. The 2 pop should be one frame long and exactly two seconds before the picture starts. Some editors also use a tail pop two seconds after the picture stops.

If a film has multiple reels, it's a good idea to have a video tape for every reel. Each video tape should have a 2 pop and time code that starts on the same hour as the reel number, example: reel 2 should start at 02:00:00:00.


Step 2:
Transfer Sound to Video Friendly Format

Location sound recorded in sync with film on a separate recorder will also need to be pulled down or slowed down .1% during the transfer process. The sound should then be transferred to a high quality digital sound format such as Time Code DAT, one of the digital 8-track formats or straight to hard drive. The sound transfer, either tape or computer sound files must have the same time code as the coinciding video. The safest way to ensure this is to pre-record (pre-stripe) the time code on the tape. Then have the audio tape machine run exactly with ("chase") the time code on the video while transferring the sound.

Another option is to carefully transfer the sound onto the audio tracks of a high quality video format for use as a reference track. If transferred properly this sound should be good enough quality to be used as part of the final mix. This can save time and money by not having to do separate audio and video transfers. It might also eliminate introducing possible sync problems between the sound tracks and video by not having them on different tapes. Whether or not you transfer the final sound mix onto your video of the film, always put some sound on it for reference.

*Standard sample rates.
**Non-standard sample. Some DAW's can work in non-standard sample rates, although after editing the sound effects and royalty free music will need to be output at one of the standard rates if it's going to stay digital.


Step 3:
Record Sound into Digital Audio Workstation

With a time code referenced audio tape that matches the edited picture or a video tape with high quality sound tracks, record or digitize the sound into the Digital Audio Workstation [DAW] of choice. To maintain proper sync make sure the DAW is referencing (chasing or acting as the master) the time code on the tape containing the sound effects and royalty free music.

If editing picture with an off-line video editor, record the sound into the editor at the highest possible quality. This will ensure the edited sound output is of sufficient quality to use as part of the final mix. Having the best possible sound tracks at this stage can save the extra step of having to re-digitize the sound effects and royalty free music and reassemble the sound edit.

If the video editor's software is compatible with the sound editor's software, the sound and the EDL can be exported from the video editor and imported into the sound editor. This will allow access to the edits already made to the sound in the video editor. Importing sound is the fastest and easiest way to get the sound effects and music into a DAW. It also saves time by allowing you access to the previous edits; you're able to pick-up where the video editor left off. With this process the sound files can be copied from the video editor's hard drive(s) to the sound editor's hard drive(s), or you can continue to work off the same hard drive. This is great, if everything is compatible; if not, it can be a waste of time and the sound will have to be transferred to tape anyway.

With either of these processes, there's no need to go back to the source tapes to rebuild the sound edit before adding the final sound effects and production music can begin.


Step 4:
Sweeten the Sound

This is the stage where most of the time will be spent. This is where the speed and flexibility of sweetening sound with a DAW makes all the previous steps worthwhile. Editing sound effects and adding music and sound effects is a breeze. Be sure to take full advantage of the features in most DAW's, such as onboard EQ, pitch shift, time compression and expansion, automated level and pan adjustments, fades and crossfades, and ease of creating alternate edits. Cutting and pasting together things like holes in the ambiance tracks and being able to work on individual words on the dialogue tracks are easy for an experienced editor.

Before you go into an audio post production studio, watch your edited film many, many times, taking notes about what will need to addressed during sound sweetening. Some of the things to look for are: what dialogue will need to be fixed and what dialogue will need to be replaced (looping/ADR), or if there are alternate takes that might work that already exist on the production tapes; what added sounds will be needed and if they will need to be foleyed, or if sound effect libraries can be used. Of course, don't forget the music. Have a very good idea of where the music is going before you start because it will affect everything else. Sitting at home and taking notes is cheaper than paying for time in a recording studio, so don't rush into it. Before you start sweetening, have pre-production meetings with the music composer and the sound designer/audio engineer. This will ensure everybody working on the film understands the creative direction, and recognizes what can and can't be accomplished within the budget and time frame.


Step 5:
Mix the Sound

Check with the theaters where the film will be played and make sure the sound format you choose will be compatible with their systems. Be it mono, multi-channel, digital surround, or encoded with Dolby A or Dolby SR noise reduction, a film can only sound as good as the system that it's played back on. The following are the most common mix formats for 16mm and 35mm film.

16mm Mono:
Standard 16mm film only has a mono optical track. The frequency response and dynamic range of an optical track is less than desirable by today's standards. The frequency response is only flat between 100Hz and 4kHz, and the dynamic range should stay between -10db and +3db VU. This means the sound quality is only about as good as an audio cassette (remember those). An optical track also has a narrow dynamic range and as a result; loud signals may either distort, or force the whole mix to be transferred at a lower level and very quiet signals could get lost. With its narrow frequency response, very low and very high frequencies can be lost. A mono mix should be created with these limitations in mind.

Dolby SR, and to a lesser degree Dolby A noise reduction, can improve the frequency response and dynamic range of optical tracks. The sound can be processed through either of these formats while mixing or while transferring the mixed sound back to film.

16mm Stereo:
If you don't have a multi-channel encoding system available to you, a full range stereo mix should be created for use with stereo video formats such as VHS, Beta or DVD. The frequency response and dynamic range of most stereo formats are far better than that of optical tracks. Theaters seldom use standard 2 channel stereo sound. Most theater playback systems use some type of multi-channel system designed to decode 4 or 5.1 channel audio mixes.

35mm:
35mm films have the availability of multiple optical tracks, which gives you several choices of mix formats. 35mm usually uses some type of noise reduction such as Dolby SR or Dolby A because of the optical tracks. The most common mix formats are 4 channel and 5.1 channel mixes.

The general rule for multi-channel mixing is to have the dialogue and the main sound effects in the center and front speakers; ambiances and secondary sound effects in the surround speakers; and the music in all of the speakers, being the loudest in the front. The specifics of where to place things in the mix are up to you.

Multi-channel mixes should be mixed through a multi-channel audio encoder like the ones made by Dolby or Ultra Stereo and in an environment designed for this. This will allow all the sound channels to be heard as they will be heard in a theater. The whole multi-channel mix is encoded onto two tracks on the film. These two tracks have the ability to play one (mono), two (stereo), or multi-channel (either 4 or 5.1) audio mixes. Once in the theater, it's decoded back into the multi-channels while being played back.


Four Channel Mix:
Sometimes called Dolby stereo, the four channel mix consists of front left and right, center, and surround (rear). This is the 'standard' or most common mix format for 35mm films on a budget. Most theaters are set up to play this type of mix.

If a multi-channel audio encoding system isn't available while actually mixing, you can cheat. A standard two channel stereo mix that hasn't been encoded for four channels can be played through a Dolby stereo decoder. Most of the mix will end up in the center and front left and right channels. Things panned full left and right will end up in the center channel; things partially panned will end up, in varying degrees, in the front left or right channels; anything with phase differences will end up in the surround channel. Thing with phase differences that could end up in the rear speaker would parts within the music or anything processed with reverb or other processors that alter phase. Also purposely reversing the phase on things can cause them to be played out of the rear speaker. This is a guessing game though, because without being able to monitor the mix through an encoder, you never really know exactly where things will end up.

5.1 Channel Mix:
5.1 is six discrete audio channels. The channels are front left and right, center, surround left and right, and sub woofer. This type of mix needs to be digitally encoded onto two channels, and is far more complicated to mix and encode, and far more expensive to access the gear needed. This is a very common format for bigger budget films. There are several variations of this mix format, SRD, DTS, and SDDS are the most popular.

Stems:
Stems are separate sub-mixes of the main elements that make up a complete mix. Generally this means creating separate mixes of Dialogue, Music, and Sound Effects (D, M and E). Stem mixes make it easy to revise a mix or replace an element such as dialogue for a foreign release. Stems also allow access to the individual elements for multi-channel mixing. It's a good idea to record stem mixes for any project, whether it's 16mm or 35mm.

Basic rules for all mixes:
A. Record the mix to a high quality digital audio format, locked to the correct time code.

B. Make sure there's an audible '2 pop', one frame long, that corresponds with the number 2 on the leader count down, exactly two seconds before the start of the picture. This may be the only sync reference when the sound is transferred to film, so its accuracy is very important.

C. The DAW you use should be capable of full time code syncing. This means it can read every time code number while constantly correcting speed and location. This is opposed to trigger sync, which reads only the first number, then runs without constant regulation of speed and location.

D. Running speed of all the video and audio machines and computers should be regulated by the same video signal known as House Sync.

E. Check the sync of the mix by watching the final picture while listening to the sound from the final mix down, not the sound tracks coming from the DAW. The DAW could be playing the mix correctly, while the sync of the mix itself could be off. This can easily happen if there's a time code or sample rate discrepancy.

F. Put a 1k tone on the head of the tape for 30 seconds which represents 0 VU. The transfer back to film is made according to this level.

G. Label the tape with all the pertinent information including SMPTE time code rate, sample rate, tone level, program length, date, title, and if any multi-channel signal encoding or noise reduction was used (these also have their own set of tones).


Step 6:
Transfer Sound to Film

For most films, the sound mix is transferred to mag full coat first. From the mag the audio is transferred to the film via optical or magnetic tracks. If the sound is going to be in sync with film it needs to be pulled up, or sped up .1% from video speed to film speed. There are two methods of doing this, either speed up the digital sound playback .1% to film speed and transfer the sound mix to a mag machine running at film speed; or slow down a mag machine .1% to video speed and transfer the sound mix at video speed. Either way, this process should be handled by a competent transfer facility experienced in working with sound for film.


Step 7:
Check the Mix

Before the final print is made, check the transferred sound mix against the film. Have the sound transferred to mag and then play that with the answer print of the film. Once the final transfer is done, check that as well. Make sure everything is in sync and sounding good; don't wait until you're sitting in a theater full of people to see if the transfer worked.




Conclusion

Digital audio is still a relatively new technology and will continue to grow and evolve. With this growth will come changes in format standards, as well as in some of the techniques used in working with digital sound for film. This booklet was written with this in mind. Some of the details covered here may change, but the general concepts should remain the same. Hope this saves you some time in the studio.



About the Author:
Soundranger Kevin Tone has won both an Emmy and Golden Reel for his work in audio post production. He has been working with audio for film and television and interactive media for over fifteen years. During this time he has taught both film and music classes as well as worked on numerous major and independent films.