Confirm that your audio interface is functioning properly.
First, Quit Pure Vinyl. Open Apple's Audio MIDI Setup application (in /Applications/Utilities) and insure that your audio interface is selected as both the Default Input and Default Output device. Try playing some music from iTunes. If the audio interface is functioning correctly, you should hear playback from iTunes.
If the interface doesn't appear in Audio MIDI Setup, there is a software or hardware fault with the interface that must be corrected before proceeding. Consult the documentation for your audio interface for assistance.
Note: there is an issue in the latest Pure Vinyl Recorder (Version 2.2) that can prevent the audio input from starting. The workaround is to quit Pure Vinyl Recorder and then launch it again. This will only happen on the very first launch, or if you delete the application's preferences file (~Library/Preferences/com.channld.PureVinylRecorder.plist). It will not occur if you've updated a previous version of Pure Vinyl. This minor issue will be corrected in a future update.
Dynamic range is just one factor that determines the quality of digital audio reproduction. Besides the advantage of using 24 bit resolution during the initial recording to provide extra headroom in setting the recording level, there's also the matter of quantization resolution.
A 16 bit digital word resolves loudness levels in steps that are 1/65,536 of the full scale signal. In other words, when an audio signal is digitized, the permissible values of the digitized result only can occur in these discrete (quantized) steps.
Digitizing a vinyl recording with 16 bit resolution forces the audio to be represented by these 1/65,536 digital loudness steps. This process occurs at every digital sample. (The total number of samples representing the waveform is determined by the sample rate, yet another factor affecting the audio quality.)
With 24 bit audio, the resolvable levels are much more closely spaced together. Instead of 65,536 possible loudness levels, there are 16,777,276. In other words, the loudness "steps" in the waveform will be 256 times finer (this isn't the same as "oversampling," which refers to the sample rate). Consequently, audio sampled at 24 bit resolution more closely resembles the original analog signal than if sampled at 16 bit resolution. Note that "32 bit" resolution, sometimes mentioned in this context, doesn't indicate the resolution of the audio interface; the 32 bits refers to the resolution of the digital stream used internally for processing by the computer. Incidentally, Pure Vinyl's internal signal processing path goes much, much further than that, by actually using 64 bits (known as double precision floating point) of resolution!
The reason for using 24 bits rather than 16 bits is just to preserve the original signal as faithfully as possible. "High rez" 24 bit 192 kHz digital audio, reproduced from a high quality audio interface, is widely regarded as being indistinguishable from the original analog signal.
It's all about preserving the signal from the vinyl recording as faithfully as possible; and more.
Unlike the Compact Disc (CD) format, vinyl is capable of reproducing frequencies well above the accepted limit of human hearing. While there is debate about whether or not (or how commonly) humans are capable of perceiving frequencies above 20 kHz, it's indisputable that the effect of the "brick wall" filter used in CD production, that sharply attenuates frequencies above the 22.05 kHz Nyquist limit, is harmful to the reproduction of audio, by introducing ringing and phase shifts that affect signal frequencies below 20 kHz. Vinyl has no such limitation on the frequency response; just the smooth roll-off imposed by the mechanical properties of the stylus; no brick-wall.
If you don't believe that vinyl "has" anything above 20 kHz, check out this example, illustrating why it's worthwhile to record / archive the audio at the highest possible sample rate:
Click above image for vinyl frequency response analysis, and comparison with CD "brickwall" 20 kHz response.
That's incorrect, and is a misconception based on a simplified view of the software-based RIAA correction process.
This subject was the focus of a technical "white paper" (click to download) presented at the October, 2007 123rd Audio Engineering Society Convention in New York. The slides (aes123slides.pdf) from the presentation also are available for download.
The paper (#P4-6, Filter Reconstruction and Program Material Characteristics Mitigating Word Length Loss in Digital Signal Processing-Based Compensation Curves Used for Playback of Analog Recordings, http://www.aes.org/events/123/papers/session.cfm?code=P4) proves that software-based correction doesn't compromise audio quality.
Briefly, the low-pass characteristic of the software RIAA correction filter functions the same as the low-pass reconstruction filter used in playback of DSD digital audio ("Super Audio CD"), where a high sample rate, one-bit stream is low-pass filtered, restoring the effective digital word length (resolution).
With software-based RIAA correction, music will typically encounter between zero (none) to one bit of word length loss in the bass, which is insignificant in comparison to the 24 bit word length of modern, high resolution digital audio, and in a frequency range where human hearing is at its least sensitive.
In fact, converting the amplified vinyl signal with the treble emphasis intact, followed by applying the correction curve in software enhances the available digital resolution in the midrange and treble, where human hearing is at its most sensitive, compared to digital conversion after RIAA correction. Pure Vinyl performs all internal signal processing at 64 bit resolution, which preserves the benefit of using the pre-emphasized signal (and, as a bonus, no distortion is added). Openness, clarity and definition in the midrange and treble are improved, compared to doing a digital conversion after the signal has been corrected with a conventional phono preamplifier. More importantly, Pure Vinyl users have reported that combining software RIAA correction with a high resolution (24 bit / 192 kHz) audio interface (and a quality gain stage) even provides noticeably improved "live" vinyl playback, compared to a conventional phonograph preamplifier, confirming our own experience.
High resolution digital conversion coupled with software RIAA equalization represents the wave of the future. This wave is inevitable, whether it's from Channel D or someone else. If you're "into vinyl," and choose not to go this way, you're really missing out on superior sound.
The difference is in just two words: sound quality.
While the iMic is a remarkable device at its price point, there naturally are limits to what can be expected from the performance of a combination microphone preamp and audio interface retailing for approximately $40. (If that were incorrect, then companies selling expensive microphone preamps and audio interfaces, running into the price range of thousands of dollars each, would go out of business. That hasn't happened.)
It's unreasonable to expect to achieve high quality vinyl playback from even a conventional phonograph preamplifier retailing for $40 (they do exist). Likewise, the many engineering hurdles to be met when creating an audio interface that achieves truly high-resolution digital audio reproduction simply cannot be overcome at that price point. If, after listening to your recordings, you find that the iMic isn't delivering the sound quality you expect (as in low noise, dynamic, crystal clear detailed sound that also doesn't cause "listening fatigue" when played at louder levels), then you should strongly consider upgrading your audio interface. The enjoyment of recorded music, whether from vinyl or other sources, always is enhanced by using better quality equipment. And if you're going to invest a lot of time into making format conversions of your vinyl, don't you want to have the best quality possible?
This also extends to the software. Pure Vinyl is designed (leveraging our more than decades-long expertise in developing audio analysis and signal processing software) from the bottom up to painstakingly preserve all of the sound quality captured by your audio interface. This goal can't be accomplished without the proper care and attention to detail.**
Here are many features of Pure Vinyl not found together in other software:
- Pristine, 64 bit double-precision floating point internal signal path for no added distortion
- Record to choice of file formats at 44.1/48/88.2/96/176.4/192 kHz, 16/24/32 bit, optional Apple Lossless Compression
- 64 bit sample rate converter (SRC), exclusive Channel D design* (see this page for comparisons with other SRCs)
- 64 bit SRC can be used to mirror to a second, CD format file, while simultaneously recording high-resolution audio
- Adjustable, noise shaped dithering for SRC / conversion to 16 bit format
- 64 bit RIAA vinyl compensation curve, for live monitoring or file playback, accurate within a small fraction of a dB
- Library with choice of dozens of customizable correction curves, for playing pre - 1955 recordings
- Adjustable Noise Gate: automatically pauses recording and locks when music stops (such as stylus lift)
- Precision signal level metering with RMS, RMS balance, peak, peak histogram and peak hold
- Controls for channel swap, mono, polarity inversion, precision balance trim
- Unique, "scratch - style" grab and spin editor with "vinyl" images rendered from the audio
- Adjustable (frequency, slope) high-pass "rumble" filter
- ZEPHRR® zero-phase rumble reduction filter for track rendering, slope up to 96 dB / octave
- Nondestructive track editor and pop and click editor, with automatic peak detection and normalization
- Split recording into tracks for making CD compilations, iTunes, etc. digital jukebox / iPod
- 64 bit internal upsampler for high-resolution (up to 192 kHz 24 bit) playback of audio tracks through high quality audio interfaces
- Fun, animated spinning "virtual vinyl" image during playback re-creates the experience of seeing that spiral-scratch disc spin
- Adjustable (frequency, slope, time alignment, etc.) crossover for audio playback on high-end (bi-amped) audio systems
- Bounce feature for distributing high-resolution (up to 192 kHz / 24 bit) audio to remote computers via Ethernet
- Includes a free activation code for Channel D's AudioLeak 2 audio loudness analyzer
Note: all updates to Pure Vinyl have been provided free of charge, even to those who purchased an activation code for Version 1.0, way back in mid-2006.
*Other software running on the Mac may use Apple's QuickTime routines for sample rate conversion (SRC) / pitch alteration, etc. While that's easier on the software developer that chooses to use QuickTime, the quality of QuickTime's audio conversion routines have been regarded unfavorably, even in Apple - sanctioned audio forums. The AudioUnit SRCs available in Mac OS X 10.5 Leopard provide an improvement in quality over QuickTime; see tests of SRC software on our site and elsewhere. Despite this, we still use our own well-tested SRC design.
**Everything affecting the audio quality of Pure Vinyl has been carefully designed and validated; we don't just "bolt together" generic pieces of audio software from other sources.
Pure Vinyl Recorder uses a large sound input buffer that causes the "latency" you are experiencing. A large sound buffer serves as extra insurance from skips in the sound as the recording is saved to disk. It is not cause for concern and doesn't harm the recording quality.
This amount of latency might be a problem for multichannel studio recording (overdubbing), but doesn't cause problems with two-channel audio transferred from pre-recorded sources (or when recording live sources without needing overdubbing).
A side benefit of the latency is that it will decrease the susceptibility of your sound system to audio feedback between the loudspeakers and turntable, if monitoring a vinyl recording at a high listening volume.
From your email you've indicated that you're running Mac OS X 10.4.10 on an Intel Processor Mac. There is a known problem with this OS X update on Intel processor Macs, causing audio devices to stop working. This problem isn't caused by Pure Vinyl.
The solution is to update to Mac OS X 10.4.11 or later, or to download and install the OS X 10.4.10 v1.1 Combo Update from Apple from their OS support downloads page:
http://www.apple.com/support/downloads/macosx10410comboupdatev11intel.html